Variable names in Perl can have several formats. Usually, theymust begin with a letter or underscore, in which case they can bearbitrarily long (up to an internal limit of 251 characters) andmay contain letters, digits, underscores, or the special sequence:: or '. In this case, the part before the last :: or' is taken to be a package qualifier; see perlmod.

Perl variable names may also be a sequence of digits or a singlepunctuation or control character. These names are all reserved forspecial uses by Perl; for example, the all-digits names are usedto hold data captured by backreferences after a regular expressionmatch. Perl has a special syntax for the single-control-characternames: It understands ^X (caret X) to mean the control-Xcharacter. For example, the notation $^W (dollar-sign caretW) is the scalar variable whose name is the single charactercontrol-W. This is better than typing a literal control-Winto your program.

Since Perl 5.6, Perl variable names may be alphanumericstrings that begin with control characters (or better yet, a caret).These variables must be written in the form ${^Foo}; the bracesare not optional. ${^Foo} denotes the scalar variable whosename is a control-F followed by two o's. These variables arereserved for future special uses by Perl, except for the ones thatbegin with ^_ (control-underscore or caret-underscore). Nocontrol-character name that begins with ^_ will acquire a specialmeaning in any future version of Perl; such names may therefore beused safely in programs. $^_ itself, however, is reserved.

Perl identifiers that begin with digits, control characters, orpunctuation characters are exempt from the effects of the packagedeclaration and are always forced to be in package main; they arealso exempt from strict 'vars' errors. A few other names are alsoexempt in these ways:

 ENV  STDIN
  INC  STDOUT
  ARGV STDERR
  ARGVOUT
  SIG

In particular, the special ${^_XYZ} variables are always takento be in package main, regardless of any package declarationspresently in scope.

SPECIAL VARIABLES

The following names have special meaning to Perl. Most punctuationnames have reasonable mnemonics, or analogs in the shells.Nevertheless, if you wish to use long variable names, you need only say:

 use English;

at the top of your program. This aliases all the short names to the longnames in the current package. Some even have medium names, generallyborrowed from awk. To avoid a performance hit, if you don't need the$PREMATCH, $MATCH, or $POSTMATCH it's best to use the Englishmodule without them:

 use English '-no_match_vars';

Before you continue, note the sort order for variables. In general, wefirst list the variables in case-insensitive, almost-lexigraphicalorder (ignoring the { or ^ preceding words, as in ${^UNICODE}or $^T), although $_ and @_ move up to the top of the pile.For variables with the same identifier, we list it in order of scalar,array, hash, and bareword.

General Variables

$ARG
$_
The default input and pattern-searching space. The following pairs areequivalent:
```
 while (<>) {...} # equivalent only in while!
  while (defined($_ = <>)) {...}
 
  /^Subject:/
  $_ =~ /^Subject:/
 
  tr/a-z/A-Z/
  $_ =~ tr/a-z/A-Z/
 
  chomp
  chomp($_)
 
```
Here are the places where Perl will assume $_ even if you don't use it:
- The following functions use $_ as a default argument:
  abs, alarm, chomp, chop, chr, chroot,cos, defined, eval, evalbytes, exp, glob,hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print,quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only),rmdir, sin, split (on its second argument), sqrt, stat, study, uc, ucfirst,unlink, unpack.
- All file tests (-f, -d) except for -t, which defaults to STDIN.See -X
- The pattern matching operations m//, s/// and tr/// (aka y///)when used without an =~ operator.
- The default iterator variable in a foreach loop if no othervariable is supplied.
- The implicit iterator variable in the grep() and map() functions.
- The implicit variable of given().
- The default place to put an input record when a <FH>operation's result is tested by itself as the sole criterion of a whiletest. Outside a while test, this will not happen.
As $_ is a global variable, this may lead in some cases to unwantedside-effects. As of perl 5.10, you can now use a lexical version of$_ by declaring it in a file or in a block with my. Moreover,declaring our $_ restores the global $_ in the current scope.
Mnemonic: underline is understood in certain operations.
@ARG
@_
Within a subroutine the array @_ contains the parameters passed tothat subroutine. Inside a subroutine, @_ is the default array forthe array operators push, pop, shift, and unshift.
See perlsub.
$LIST_SEPARATOR
$"
When an array or an array slice is interpolated into a double-quotedstring or a similar context such as /.../, its elements areseparated by this value. Default is a space. For example, this:
```
 print "The array is: @array\n";
 
```
is equivalent to this:
```
 print "The array is: " . join($", @array) . "\n";
 
```
Mnemonic: works in double-quoted context.
$PROCESS_ID
$PID
$$
The process number of the Perl running this script. Though you can setthis variable, doing so is generally discouraged, although it can beinvaluable for some testing purposes. It will be reset automaticallyacross fork() calls.
Note for Linux and Debian GNU/kFreeBSD users: Before Perl v5.16.0 perlwould emulate POSIX semantics on Linux systems using LinuxThreads, apartial implementation of POSIX Threads that has since been supersededby the Native POSIX Thread Library (NPTL).
LinuxThreads is now obsolete on Linux, and and caching getpid()like this made embedding perl unnecessarily complex (since you'd haveto manually update the value of $$), so now $$ and getppid()will always return the same values as the underlying C library.
Debian GNU/kFreeBSD systems also used LinuxThreads up until andincluding the 6.0 release, but after that moved to FreeBSD threadsemantics, which are POSIX-like.
To see if your system is affected by this discrepancy check ifgetconf GNU_LIBPTHREAD_VERSION | grep -q NPTL returns a falsevalue. NTPL threads preserve the POSIX semantics.
Mnemonic: same as shells.
$PROGRAM_NAME
$0
Contains the name of the program being executed.
On some (but not all) operating systems assigning to $0 modifiesthe argument area that the ps program sees. On some platforms youmay have to use special ps options or a different ps to see thechanges. Modifying the $0 is more useful as a way of indicating thecurrent program state than it is for hiding the program you'rerunning.
Note that there are platform-specific limitations on the maximumlength of $0. In the most extreme case it may be limited to thespace occupied by the original $0.
In some platforms there may be arbitrary amount of padding, forexample space characters, after the modified name as shown by ps.In some platforms this padding may extend all the way to the originallength of the argument area, no matter what you do (this is the casefor example with Linux 2.2).
Note for BSD users: setting $0 does not completely remove "perl"from the ps(1) output. For example, setting $0 to "foobar" mayresult in "perl: foobar (perl)" (whether both the "perl: " prefixand the " (perl)" suffix are shown depends on your exact BSD variantand version). This is an operating system feature, Perl cannot help it.
In multithreaded scripts Perl coordinates the threads so that anythread may modify its copy of the $0 and the change becomes visibleto ps(1) (assuming the operating system plays along). Note thatthe view of $0 the other threads have will not change since theyhave their own copies of it.
If the program has been given to perl via the switches -e or -E,$0 will contain the string "-e".
On Linux as of perl 5.14 the legacy process name will be set withprctl(2), in addition to altering the POSIX name via argv[0] asperl has done since version 4.000. Now system utilities that read thelegacy process name such as ps, top and killall will recognize thename you set when assigning to $0. The string you supply will becut off at 16 bytes, this is a limitation imposed by Linux.
Mnemonic: same as sh and ksh.
$REAL_GROUP_ID
$GID
$(
The real gid of this process. If you are on a machine that supportsmembership in multiple groups simultaneously, gives a space separatedlist of groups you are in. The first number is the one returned bygetgid(), and the subsequent ones by getgroups(), one of which may bethe same as the first number.
However, a value assigned to $( must be a single number used toset the real gid. So the value given by $( should not be assignedback to $( without being forced numeric, such as by adding zero. Notethat this is different to the effective gid ($)) which does take alist.
You can change both the real gid and the effective gid at the sametime by using POSIX::setgid(). Changesto $( require a check to $!to detect any possible errors after an attempted change.
Mnemonic: parentheses are used to group things. The real gid is thegroup you left, if you're running setgid.
$EFFECTIVE_GROUP_ID
$EGID
$)
The effective gid of this process. If you are on a machine thatsupports membership in multiple groups simultaneously, gives a spaceseparated list of groups you are in. The first number is the onereturned by getegid(), and the subsequent ones by getgroups(),one of which may be the same as the first number.
Similarly, a value assigned to $) must also be a space-separatedlist of numbers. The first number sets the effective gid, andthe rest (if any) are passed to setgroups(). To get the effect of anempty list for setgroups(), just repeat the new effective gid; that is,to force an effective gid of 5 and an effectively empty setgroups()list, say $) = "5 5" .
You can change both the effective gid and the real gid at the sametime by using POSIX::setgid() (use only a single numeric argument).Changes to $) require a check to $! to detect any possible errorsafter an attempted change.
$<, $>, $( and $) can be set only onmachines that support the corresponding set[re][ug]id() routine. $(and $) can be swapped only on machines supporting setregid().
Mnemonic: parentheses are used to group things. The effective gidis the group that's right for you, if you're running setgid.
$REAL_USER_ID
$UID
$<
The real uid of this process. You can change both the real uid and theeffective uid at the same time by using POSIX::setuid(). Sincechanges to $< require a system call, check $! after a changeattempt to detect any possible errors.
Mnemonic: it's the uid you came from, if you're running setuid.
$EFFECTIVE_USER_ID
$EUID
$>
The effective uid of this process. For example:
```
 $< = $>; # set real to effective uid
  ($<,$>) = ($>,$<);  # swap real and effective uids
 
```
You can change both the effective uid and the real uid at the sametime by using POSIX::setuid(). Changes to $> require a checkto $! to detect any possible errors after an attempted change.
$< and $> can be swapped only on machinessupporting setreuid().
Mnemonic: it's the uid you went to, if you're running setuid.
$SUBSCRIPT_SEPARATOR
$SUBSEP
$;
The subscript separator for multidimensional array emulation. If yourefer to a hash element as
```
 $foo{$a,$b,$c}
 
```
it really means
```
 $foo{join($;, $a, $b, $c)}
 
```
But don't put
```
 @foo{$a,$b,$c}# a slice--note the @
 
```
which means
```
 ($foo{$a},$foo{$b},$foo{$c})
 
```
Default is "\034", the same as SUBSEP in awk. If your keys containbinary data there might not be any safe value for $;.
Consider using "real" multidimensional arrays as describedin perllol.
Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
$a
$b
Special package variables when using sort(), see sort.Because of this specialness $a and $b don't need to be declared(using use vars, or our()) even when using the strict 'vars'pragma. Don't lexicalize them with my $a or my $b if you want tobe able to use them in the sort() comparison block or function.
%ENV
The hash %ENV contains your current environment. Setting avalue in ENV changes the environment for any child processesyou subsequently fork() off.
$SYSTEM_FD_MAX
$^F
The maximum system file descriptor, ordinarily 2. System filedescriptors are passed to exec()ed processes, while higher filedescriptors are not. Also, during anopen(), system file descriptors arepreserved even if the open() fails (ordinary file descriptors areclosed before the open() is attempted). The close-on-execstatus of a file descriptor will be decided according to the value of$^F when the corresponding file, pipe, or socket was opened, not thetime of the exec().
@F
The array @F contains the fields of each line read in when autosplitmode is turned on. See perlrun for the -a switch. This arrayis package-specific, and must be declared or given a full package nameif not in package main when running under strict 'vars'.
@INC
The array @INC contains the list of places that the do EXPR,require, or use constructs look for their library files. Itinitially consists of the arguments to any -I command-lineswitches, followed by the default Perl library, probably/usr/local/lib/perl, followed by ".", to represent the currentdirectory. ("." will not be appended if taint checks are enabled,either by -T or by -t.) If you need to modify this at runtime,you should use the use lib pragma to get the machine-dependentlibrary properly loaded also:
```
 use lib '/mypath/libdir/';
  use SomeMod;
 
```
You can also insert hooks into the file inclusion system by putting Perlcode directly into @INC. Those hooks may be subroutine references,array references or blessed objects. See require for details.
%INC
The hash %INC contains entries for each filename included via thedo, require, or use operators. The key is the filenameyou specified (with module names converted to pathnames), and thevalue is the location of the file found. The requireoperator uses this hash to determine whether a particular file hasalready been included.
If the file was loaded via a hook (e.g. a subroutine reference, seerequire for a description of these hooks), this hook isby default inserted into %INC in place of a filename. Note, however,that the hook may have set the %INC entry by itself to provide some morespecific info.
$INPLACE_EDIT
$^I
The current value of the inplace-edit extension. Use undef to disableinplace editing.
Mnemonic: value of -i switch.
$^M
By default, running out of memory is an untrappable, fatal error.However, if suitably built, Perl can use the contents of $^Mas an emergency memory pool after die()ing. Suppose that your Perlwere compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.Then
```
 $^M = 'a' x (1 << 16);
 
```
would allocate a 64K buffer for use in an emergency. See theINSTALL file in the Perl distribution for information on how toadd custom C compilation flags when compiling perl. To discourage casualuse of this advanced feature, there is no English long name forthis variable.
This variable was added in Perl 5.004.
$OSNAME
$^O
The name of the operating system under which this copy of Perl wasbuilt, as determined during the configuration process. For examplessee PLATFORMS in perlport.
The value is identical to $Config{'osname'}. See also Configand the -V command-line switch documented in perlrun.
In Windows platforms, $^O is not very helpful: since it is alwaysMSWin32, it doesn't tell the difference between95/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() orWin32::GetOSVersion() (see Win32 and perlport) to distinguishbetween the variants.
This variable was added in Perl 5.003.
%SIG
The hash %SIG contains signal handlers for signals. For example:
```
 sub handler {   # 1st argument is signal name
 my($sig) = @_;
 print "Caught a SIG$sig--shutting down\n";
 close(LOG);
 exit(0);
 }
 
  $SIG{'INT'}  = \&handler;
  $SIG{'QUIT'} = \&handler;
  ...
  $SIG{'INT'}  = 'DEFAULT';   # restore default action
  $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
 
```
Using a value of 'IGNORE' usually has the effect of ignoring thesignal, except for the CHLD signal. See perlipc for more aboutthis special case.
Here are some other examples:
```
 $SIG{"PIPE"} = "Plumber";   # assumes main::Plumber (not
 # recommended)
  $SIG{"PIPE"} = \&Plumber;   # just fine; assume current
 # Plumber
  $SIG{"PIPE"} = *Plumber; # somewhat esoteric
  $SIG{"PIPE"} = Plumber();   # oops, what did Plumber()
 # return??
 
```
Be sure not to use a bareword as the name of a signal handler,lest you inadvertently call it.
If your system has the sigaction() function then signal handlersare installed using it. This means you get reliable signal handling.
The default delivery policy of signals changed in Perl 5.8.0 fromimmediate (also known as "unsafe") to deferred, also known as "safesignals". See perlipc for more information.
Certain internal hooks can be also set using the %SIG hash. Theroutine indicated by $SIG{__WARN__} is called when a warningmessage is about to be printed. The warning message is passed as thefirst argument. The presence of a __WARN__ hook causes theordinary printing of warnings to STDERR to be suppressed. You canuse this to save warnings in a variable, or turn warnings into fatalerrors, like this:
```
 local $SIG{__WARN__} = sub { die $_[0] };
  eval $proggie;
 
```
As the 'IGNORE' hook is not supported by __WARN__, you candisable warnings using the empty subroutine:
```
 local $SIG{__WARN__} = sub {};
 
```
The routine indicated by $SIG{__DIE__} is called when a fatalexception is about to be thrown. The error message is passed as thefirst argument. When a __DIE__ hook routine returns, the exceptionprocessing continues as it would have in the absence of the hook,unless the hook routine itself exits via a goto &sub, a loop exit,or a die(). The __DIE__ handler is explicitly disabled duringthe call, so that you can die from a __DIE__ handler. Similarlyfor __WARN__.
Due to an implementation glitch, the $SIG{__DIE__} hook is calledeven inside an eval(). Do not use this to rewrite a pendingexception in $@, or as a bizarre substitute for overridingCORE::GLOBAL::die(). This strange action at a distance may be fixedin a future release so that $SIG{__DIE__} is only called if yourprogram is about to exit, as was the original intent. Any other use isdeprecated.
__DIE__/__WARN__ handlers are very special in one respect: theymay be called to report (probable) errors found by the parser. In sucha case the parser may be in inconsistent state, so any attempt toevaluate Perl code from such a handler will probably result in asegfault. This means that warnings or errors that result from parsingPerl should be used with extreme caution, like this:
```
 require Carp if defined $^S;
  Carp::confess("Something wrong") if defined &Carp::confess;
  die "Something wrong, but could not load Carp to give "
   . "backtrace...\n\t"
   . "To see backtrace try starting Perl with -MCarp switch";
 
```
Here the first line will load Carp unless it is the parser whocalled the handler. The second line will print backtrace and die ifCarp was available. The third line will be executed only if Carp wasnot available.
Having to even think about the $^S variable in your exceptionhandlers is simply wrong. $SIG{__DIE__} as currently implementedinvites grievous and difficult to track down errors. Avoid itand use an END{} or CORE::GLOBAL::die override instead.
See die, warn, eval, andwarnings for additional information.
$BASETIME
$^T
The time at which the program began running, in seconds since theepoch (beginning of 1970). The values returned by the -M, -A,and -C filetests are based on this value.
$PERL_VERSION
$^V
The revision, version, and subversion of the Perl interpreter,represented as a version object.
This variable first appeared in perl 5.6.0; earlier versions of perlwill see an undefined value. Before perl 5.10.0 $^V was representedas a v-string.
$^V can be used to determine whether the Perl interpreter executinga script is in the right range of versions. For example:
```
 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
 
```
To convert $^V into its string representation use sprintf()'s"%vd" conversion:
```
 printf "version is v%vd\n", $^V;  # Perl's version
 
```
See the documentation of use VERSION and require VERSIONfor a convenient way to fail if the running Perl interpreter is too old.
See also $] for an older representation of the Perl version.
This variable was added in Perl 5.6.
Mnemonic: use ^V for Version Control.
${^WIN32_SLOPPY_STAT}
If this variable is set to a true value, then stat() on Windows willnot try to open the file. This means that the link count cannot bedetermined and file attributes may be out of date if additionalhardlinks to the file exist. On the other hand, not opening the fileis considerably faster, especially for files on network drives.
This variable could be set in the sitecustomize.pl file toconfigure the local Perl installation to use "sloppy" stat() bydefault. See the documentation for -f inperlrun for more information about sitecustomization.
This variable was added in Perl 5.10.
$EXECUTABLE_NAME
$^X
The name used to execute the current copy of Perl, from C'sargv[0] or (where supported) /proc/self/exe.
Depending on the host operating system, the value of $^X may bea relative or absolute pathname of the perl program file, or maybe the string used to invoke perl but not the pathname of theperl program file. Also, most operating systems permit invokingprograms that are not in the PATH environment variable, so thereis no guarantee that the value of $^X is in PATH. For VMS, thevalue may or may not include a version number.
You usually can use the value of $^X to re-invoke an independentcopy of the same perl that is currently running, e.g.,
```
 @first_run = `$^X -le "print int rand 100 for 1..100"`;
 
```
But recall that not all operating systems support forking orcapturing of the output of commands, so this complex statementmay not be portable.
It is not safe to use the value of $^X as a path name of a file,as some operating systems that have a mandatory suffix onexecutable files do not require use of the suffix when invokinga command. To convert the value of $^X to a path name, use thefollowing statements:
```
 # Build up a set of file names (not command names).
  use Config;
  my $this_perl = $^X;
  if ($^O ne 'VMS') {
 $this_perl .= $Config{_exe}
   unless $this_perl =~ m/$Config{_exe}$/i;
 }
 
```
Because many operating systems permit anyone with read access tothe Perl program file to make a copy of it, patch the copy, andthen execute the copy, the security-conscious Perl programmershould take care to invoke the installed copy of perl, not thecopy referenced by $^X. The following statements accomplishthis goal, and produce a pathname that can be invoked as acommand or referenced as a file.
```
 use Config;
  my $secure_perl_path = $Config{perlpath};
  if ($^O ne 'VMS') {
 $secure_perl_path .= $Config{_exe}
  unless $secure_perl_path =~ m/$Config{_exe}$/i;
 }
 
```

Variables related to regular expressions

Most of the special variables related to regular expressions are sideeffects. Perl sets these variables when it has a successful match, soyou should check the match result before using them. For instance:

 if( /P(A)TT(ER)N/ ) {
 print "I found $1 and $2\n";
 }

These variables are read-only and dynamically-scoped, unless we noteotherwise.

The dynamic nature of the regular expression variables means thattheir value is limited to the block that they are in, as demonstratedby this bit of code:

 my $outer = 'Wallace and Grommit';
  my $inner = 'Mutt and Jeff';
 
  my $pattern = qr/(\S+) and (\S+)/;
 
  sub show_n { print "\$1 is $1; \$2 is $2\n" }
 
  {
  OUTER:
 show_n() if $outer =~ m/$pattern/;
 
 INNER: {
  show_n() if $inner =~ m/$pattern/;
  }
 
 show_n();
  }

The output shows that while in the OUTER block, the values of $1and $2 are from the match against $outer. Inside the INNERblock, the values of $1 and $2 are from the match against$inner, but only until the end of the block (i.e. the dynamicscope). After the INNER block completes, the values of $1 and$2 return to the values for the match against $outer even thoughwe have not made another match:

 $1 is Wallace; $2 is Grommit
  $1 is Mutt; $2 is Jeff
  $1 is Wallace; $2 is Grommit

Due to an unfortunate accident of Perl's implementation, useEnglish imposes a considerable performance penalty on all regularexpression matches in a program because it uses the $`, $&, and$', regardless of whether they occur in the scope of useEnglish. For that reason, saying use English in libraries isstrongly discouraged unless you import it without the match variables:

 use English '-no_match_vars'

The Devel::NYTProf and Devel::FindAmpersandmodules can help you find uses of theseproblematic match variables in your code.

Since Perl 5.10, you can use the /p match operator flag and the${^PREMATCH}, ${^MATCH}, and ${^POSTMATCH} variables insteadso you only suffer the performance penalties.

$<digits> ($1, $2, ...)
Contains the subpattern from the corresponding set of capturingparentheses from the last successful pattern match, not counting patternsmatched in nested blocks that have been exited already.
These variables are read-only and dynamically-scoped.
Mnemonic: like \digits.
$MATCH
$&
The string matched by the last successful pattern match (not countingany matches hidden within a BLOCK or eval() enclosed by the currentBLOCK).
The use of this variable anywhere in a program imposes a considerableperformance penalty on all regular expression matches. To avoid thispenalty, you can extract the same substring by using @-. Startingwith Perl 5.10, you can use the /p match flag and the ${^MATCH}variable to do the same thing for particular match operations.
This variable is read-only and dynamically-scoped.
Mnemonic: like & in some editors.
${^MATCH}
This is similar to $& ($MATCH) except that it does not incur theperformance penalty associated with that variable, and is only guaranteedto return a defined value when the pattern was compiled or executed withthe /p modifier.
This variable was added in Perl 5.10.
This variable is read-only and dynamically-scoped.
$PREMATCH
$`
The string preceding whatever was matched by the last successfulpattern match, not counting any matches hidden within a BLOCK or evalenclosed by the current BLOCK.
The use of this variable anywhere in a program imposes a considerableperformance penalty on all regular expression matches. To avoid thispenalty, you can extract the same substring by using @-. Startingwith Perl 5.10, you can use the /p match flag and the${^PREMATCH} variable to do the same thing for particular matchoperations.
This variable is read-only and dynamically-scoped.
Mnemonic: ` often precedes a quoted string.
${^PREMATCH}
This is similar to $` ($PREMATCH) except that it does not incur theperformance penalty associated with that variable, and is only guaranteedto return a defined value when the pattern was compiled or executed withthe /p modifier.
This variable was added in Perl 5.10
This variable is read-only and dynamically-scoped.
$POSTMATCH
$'
The string following whatever was matched by the last successfulpattern match (not counting any matches hidden within a BLOCK or eval()enclosed by the current BLOCK). Example:
```
 local $_ = 'abcdefghi';
  /def/;
  print "$`:$&:$'\n";  # prints abc:def:ghi
 
```
The use of this variable anywhere in a program imposes a considerableperformance penalty on all regular expression matches.To avoid this penalty, you can extract the same substring byusing @-. Starting with Perl 5.10, you can use the /p match flagand the ${^POSTMATCH} variable to do the same thing for particularmatch operations.
This variable is read-only and dynamically-scoped.
Mnemonic: ' often follows a quoted string.
${^POSTMATCH}
This is similar to $' ($POSTMATCH) except that it does not incur theperformance penalty associated with that variable, and is only guaranteedto return a defined value when the pattern was compiled or executed withthe /p modifier.
This variable was added in Perl 5.10.
This variable is read-only and dynamically-scoped.
$LAST_PAREN_MATCH
$+
The text matched by the last bracket of the last successful search pattern.This is useful if you don't know which one of a set of alternative patternsmatched. For example:
```
 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
 
```
This variable is read-only and dynamically-scoped.
Mnemonic: be positive and forward looking.
$LAST_SUBMATCH_RESULT
$^N
The text matched by the used group most-recently closed (i.e. the groupwith the rightmost closing parenthesis) of the last successful searchpattern.
This is primarily used inside (?{...}) blocks for examining textrecently matched. For example, to effectively capture text to a variable(in addition to $1, $2, etc.), replace (...) with
```
 (?:(...)(?{ $var = $^N }))
 
```
By setting and then using $var in this way relieves you from having toworry about exactly which numbered set of parentheses they are.
This variable was added in Perl 5.8.
Mnemonic: the (possibly) Nested parenthesis that most recently closed.
@LAST_MATCH_END
@+
This array holds the offsets of the ends of the last successfulsubmatches in the currently active dynamic scope. $+[0] isthe offset into the string of the end of the entire match. Thisis the same value as what the pos function returns when calledon the variable that was matched against. The nth elementof this array holds the offset of the nth submatch, so$+[1] is the offset past where $1 ends, $+[2] the offsetpast where $2 ends, and so on. You can use $#+ to determinehow many subgroups were in the last successful match. See theexamples given for the @- variable.
This variable was added in Perl 5.6.
%LAST_PAREN_MATCH
%+
Similar to @+, the %+ hash allows access to the named capturebuffers, should they exist, in the last successful match in thecurrently active dynamic scope.
For example, $+{foo} is equivalent to $1 after the following match:
```
 'foo' =~ /(?<foo>foo)/;
 
```
The keys of the %+ hash list only the names of buffers that havecaptured (and that are thus associated to defined values).
The underlying behaviour of %+ is provided by theTie::Hash::NamedCapture module.
Note: %- and %+ are tied views into a common internal hashassociated with the last successful regular expression. Therefore mixingiterative access to them via each may have unpredictable results.Likewise, if the last successful match changes, then the results may besurprising.
This variable was added in Perl 5.10.
This variable is read-only and dynamically-scoped.
@LAST_MATCH_START
@-
$-[0] is the offset of the start of the last successful match.$-[n] is the offset of the start of the substring matched byn-th subpattern, or undef if the subpattern did not match.
Thus, after a match against $_, $& coincides with substr $_, $-[0],$+[0] - $-[0]. Similarly, $n coincides with substr $_, $-[n],$+[n] - $-[n] if $-[n] is defined, and $+ coincides withsubstr $_, $-[$#-], $+[$#-] - $-[$#-]. One can use $#- to find thelast matched subgroup in the last successful match. Contrast with$#+, the number of subgroups in the regular expression. Comparewith @+.
This array holds the offsets of the beginnings of the lastsuccessful submatches in the currently active dynamic scope.$-[0] is the offset into the string of the beginning of theentire match. The nth element of this array holds the offsetof the nth submatch, so $-[1] is the offset where $1begins, $-[2] the offset where $2 begins, and so on.
After a match against some variable $var:
- $` is the same as substr($var, 0, $-[0])
- $& is the same as substr($var, $-[0], $+[0] - $-[0])
- $' is the same as substr($var, $+[0])
- $1 is the same as substr($var, $-[1], $+[1] - $-[1])
- $2 is the same as substr($var, $-[2], $+[2] - $-[2])
- $3 is the same as substr($var, $-[3], $+[3] - $-[3])
This variable was added in Perl 5.6.
%LAST_MATCH_START
%-
Similar to %+, this variable allows access to the named capture groupsin the last successful match in the currently active dynamic scope. Toeach capture group name found in the regular expression, it associates areference to an array containing the list of values captured by allbuffers with that name (should there be several of them), in the orderwhere they appear.
Here's an example:
```
 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
  foreach my $bufname (sort keys %-) {
  my $ary = $-{$bufname};
  foreach my $idx (0..$#$ary) {
  print "\$-{$bufname}[$idx] : ",
   (defined($ary->[$idx])
   ? "'$ary->[$idx]'"
   : "undef"),
   "\n";
  }
  }
  }
 
```
would print out:
```
 $-{A}[0] : '1'
  $-{A}[1] : '3'
  $-{B}[0] : '2'
  $-{B}[1] : '4'
 
```
The keys of the %- hash correspond to all buffer names found inthe regular expression.
The behaviour of %- is implemented via theTie::Hash::NamedCapture module.
Note: %- and %+ are tied views into a common internal hashassociated with the last successful regular expression. Therefore mixingiterative access to them via each may have unpredictable results.Likewise, if the last successful match changes, then the results may besurprising.
This variable was added in Perl 5.10
This variable is read-only and dynamically-scoped.
$LAST_REGEXP_CODE_RESULT
$^R
The result of evaluation of the last successful (?{ code })regular expression assertion (see perlre). May be written to.
This variable was added in Perl 5.005.
${^RE_DEBUG_FLAGS}
The current value of the regex debugging flags. Set to 0 for no debug outputeven when the re 'debug' module is loaded. See re for details.
This variable was added in Perl 5.10.
${^RE_TRIE_MAXBUF}
Controls how certain regex optimisations are applied and how much memory theyutilize. This value by default is 65536 which corresponds to a 512kBtemporary cache. Set this to a higher value to tradememory for speed when matching large alternations. Setit to a lower value if you want the optimisations tobe as conservative of memory as possible but still occur, and set it to anegative value to prevent the optimisation and conserve the most memory.Under normal situations this variable should be of no interest to you.
This variable was added in Perl 5.10.

Variables related to filehandles

Variables that depend on the currently selected filehandle may be setby calling an appropriate object method on the IO::Handle object,although this is less efficient than using the regular built-invariables. (Summary lines below for this contain the word HANDLE.)First you must say

 use IO::Handle;

after which you may use either

 method HANDLE EXPR

or more safely,

 HANDLE->method(EXPR)

Each method returns the old value of the IO::Handle attribute. Themethods each take an optional EXPR, which, if supplied, specifies thenew value for the IO::Handle attribute in question. If notsupplied, most methods do nothing to the current value--except forautoflush(), which will assume a 1 for you, just to be different.

Because loading in the IO::Handle class is an expensive operation,you should learn how to use the regular built-in variables.

A few of these variables are considered "read-only". This means thatif you try to assign to this variable, either directly or indirectlythrough a reference, you'll raise a run-time exception.

You should be very careful when modifying the default values of mostspecial variables described in this document. In most cases you wantto localize these variables before changing them, since if you don't,the change may affect other modules which rely on the default valuesof the special variables that you have changed. This is one of thecorrect ways to read the whole file at once:

 open my $fh, "<", "foo" or die $!;
  local $/; # enable localized slurp mode
  my $content = <$fh>;
  close $fh;

But the following code is quite bad:

 open my $fh, "<", "foo" or die $!;
  undef $/; # enable slurp mode
  my $content = <$fh>;
  close $fh;

since some other module, may want to read data from some file in thedefault "line mode", so if the code we have just presented has beenexecuted, the global value of $/ is now changed for any other coderunning inside the same Perl interpreter.

Usually when a variable is localized you want to make sure that thischange affects the shortest scope possible. So unless you are alreadyinside some short {} block, you should create one yourself. Forexample:

 my $content = '';
  open my $fh, "<", "foo" or die $!;
  {
 local $/;
 $content = <$fh>;
  }
  close $fh;

Here is an example of how your own code can go broken:

 for ( 1..3 ){
 $\ = "\r\n";
 nasty_break();
 print "$_";
  }
 
  sub nasty_break {
 $\ = "\f";
 # do something with $_
  }

You probably expect this code to print the equivalent of

 "1\r\n2\r\n3\r\n"

but instead you get:

 "1\f2\f3\f"

Why? Because nasty_break() modifies $\ without localizing itfirst. The value you set in nasty_break() is still there when youreturn. The fix is to add local() so the value doesn't leak out ofnasty_break():

 local $\ = "\f";

It's easy to notice the problem in such a short example, but in morecomplicated code you are looking for trouble if you don't localizechanges to the special variables.

$ARGV
Contains the name of the current file when reading from <>.
@ARGV
The array @ARGV contains the command-line arguments intended forthe script. $#ARGV is generally the number of arguments minusone, because $ARGV[0] is the first argument, not the program'scommand name itself. See $0 for the command name.
ARGV
The special filehandle that iterates over command-line filenames in@ARGV. Usually written as the null filehandle in the angle operator<>. Note that currently ARGV only has its magical effectwithin the <> operator; elsewhere it is just a plain filehandlecorresponding to the last file opened by <>. In particular,passing \*ARGV as a parameter to a function that expects a filehandlemay not cause your function to automatically read the contents of all thefiles in @ARGV.
ARGVOUT
The special filehandle that points to the currently open output filewhen doing edit-in-place processing with -i. Useful when you haveto do a lot of inserting and don't want to keep modifying $_. Seeperlrun for the -i switch.
Handle->output_field_separator( EXPR )
$OUTPUT_FIELD_SEPARATOR
$OFS
$,
The output field separator for the print operator. If defined, thisvalue is printed between each of print's arguments. Default is undef.
Mnemonic: what is printed when there is a "," in your print statement.
HANDLE->input_line_number( EXPR )
$INPUT_LINE_NUMBER
$NR
$.
Current line number for the last filehandle accessed.
Each filehandle in Perl counts the number of lines that have been readfrom it. (Depending on the value of $/, Perl's idea of whatconstitutes a line may not match yours.) When a line is read from afilehandle (via readline() or <>), or when tell() orseek() is called on it, $. becomes an alias to the line counterfor that filehandle.
You can adjust the counter by assigning to $., but this will notactually move the seek pointer. Localizing $. will not localizethe filehandle's line count. Instead, it will localize perl's notionof which filehandle $. is currently aliased to.
$. is reset when the filehandle is closed, but not when an openfilehandle is reopened without an intervening close(). For moredetails, see I/O Operators in perlop. Because <> never doesan explicit close, line numbers increase across ARGV files (but seeexamples in eof).
You can also use HANDLE->input_line_number(EXPR) to access theline counter for a given filehandle without having to worry aboutwhich handle you last accessed.
Mnemonic: many programs use "." to mean the current line number.
HANDLE->input_record_separator( EXPR )
$INPUT_RECORD_SEPARATOR
$RS
$/
The input record separator, newline by default. This influences Perl'sidea of what a "line" is. Works like awk's RS variable, includingtreating empty lines as a terminator if set to the null string (anempty line cannot contain any spaces or tabs). You may set it to amulti-character string to match a multi-character terminator, or toundef to read through the end of file. Setting it to "\n\n"means something slightly different than setting to "", if the filecontains consecutive empty lines. Setting to "" will treat two ormore consecutive empty lines as a single empty line. Setting to"\n\n" will blindly assume that the next input character belongs tothe next paragraph, even if it's a newline.
```
 local $/;   # enable "slurp" mode
  local $_ = <FH>; # whole file now here
  s/\n[ \t]+/ /g;
 
```
Remember: the value of $/ is a string, not a regex. awk has tobe better for something. :-)
Setting $/ to a reference to an integer, scalar containing aninteger, or scalar that's convertible to an integer will attempt toread records instead of lines, with the maximum record size being thereferenced integer. So this:
```
 local $/ = \32768; # or \"32768", or \$var_containing_32768
  open my $fh, "<", $myfile or die $!;
  local $_ = <$fh>;
 
```
will read a record of no more than 32768 bytes from FILE. If you'renot reading from a record-oriented file (or your OS doesn't haverecord-oriented files), then you'll likely get a full chunk of datawith every read. If a record is larger than the record size you'veset, you'll get the record back in pieces. Trying to set the recordsize to zero or less will cause reading in the (rest of the) whole file.
On VMS only, record reads bypass PerlIO layers and any associatedbuffering,so you must not mix record and non-record reads on thesame filehandle. Record mode mixes with line mode only when thesame buffering layer is in use for both modes.
If you perform a record read on a FILE with an encoding layer such as:encoding(latin1) or :utf8, you may get an invalid string as aresult, may leave the FILE positioned between characters in the streamand may not be reading the number of bytes from the underlying filethat you specified. This behaviour may change without warning in afuture version of perl.
See also Newlines in perlport. Also see $..
Mnemonic: / delimits line boundaries when quoting poetry.
Handle->output_record_separator( EXPR )
$OUTPUT_RECORD_SEPARATOR
$ORS
$\
The output record separator for the print operator. If defined, thisvalue is printed after the last of print's arguments. Default is undef.
Mnemonic: you set $\ instead of adding "\n" at the end of the print.Also, it's just like $/, but it's what you get "back" from Perl.
HANDLE->autoflush( EXPR )
$OUTPUT_AUTOFLUSH
$|
If set to nonzero, forces a flush right away and after every write orprint on the currently selected output channel. Default is 0(regardless of whether the channel is really buffered by the system ornot; $| tells you only whether you've asked Perl explicitly toflush after each write). STDOUT will typically be line buffered ifoutput is to the terminal and block buffered otherwise. Setting thisvariable is useful primarily when you are outputting to a pipe orsocket, such as when you are running a Perl program under rsh andwant to see the output as it's happening. This has no effect on inputbuffering. See getc for that. See select onhow to select the output channel. See also IO::Handle.
Mnemonic: when you want your pipes to be piping hot.

Variables related to formats

The special variables for formats are a subset of those forfilehandles. See perlform for more information about Perl'sformats.

$ACCUMULATOR
$^A
The current value of the write() accumulator for format() lines.A format contains formline() calls that put their result into$^A. After calling its format, write() prints out the contentsof $^A and empties. So you never really see the contents of $^Aunless you call formline() yourself and then look at it. Seeperlform and formline PICTURE,LIST.
HANDLE->format_formfeed(EXPR)
$FORMAT_FORMFEED
$^L
What formats output as a form feed. The default is \f.
HANDLE->format_page_number(EXPR)
$FORMAT_PAGE_NUMBER
$%
The current page number of the currently selected output channel.
Mnemonic: % is page number in nroff.
HANDLE->format_lines_left(EXPR)
$FORMAT_LINES_LEFT
$-
The number of lines left on the page of the currently selected outputchannel.
Mnemonic: lines_on_page - lines_printed.
Handle->format_line_break_characters EXPR
$FORMAT_LINE_BREAK_CHARACTERS
$:
The current set of characters after which a string may be broken tofill continuation fields (starting with ^) in a format. The default is" \n-", to break on a space, newline, or a hyphen.
Mnemonic: a "colon" in poetry is a part of a line.
HANDLE->format_lines_per_page(EXPR)
$FORMAT_LINES_PER_PAGE
$=
The current page length (printable lines) of the currently selectedoutput channel. The default is 60.
Mnemonic: = has horizontal lines.
HANDLE->format_top_name(EXPR)
$FORMAT_TOP_NAME
$^
The name of the current top-of-page format for the currently selectedoutput channel. The default is the name of the filehandle with _TOPappended. For example, the default format top name for the STDOUTfilehandle is STDOUT_TOP.
Mnemonic: points to top of page.
HANDLE->format_name(EXPR)
$FORMAT_NAME
$~
The name of the current report format for the currently selectedoutput channel. The default format name is the same as the filehandlename. For example, the default format name for the STDOUTfilehandle is just STDOUT.
Mnemonic: brother to $^.

Error Variables

The variables $@, $!, $^E, and $? contain informationabout different types of error conditions that may appear duringexecution of a Perl program. The variables are shown ordered bythe "distance" between the subsystem which reported the error andthe Perl process. They correspond to errors detected by the Perlinterpreter, C library, operating system, or an external program,respectively.

To illustrate the differences between these variables, consider thefollowing Perl expression, which uses a single-quoted string. Afterexecution of this statement, perl may have set all four special errorvariables:

 eval q{
 open my $pipe, "/cdrom/install |" or die $!;
 my @res = <$pipe>
 close $pipe or die "bad pipe: $?, $!"
   };

When perl executes the eval() expression, it translates theopen(), <PIPE>, and close calls in the C run-time libraryand thence to the operating system kernel. perl sets $! tothe C library's errno if one of these calls fails.

$@ is set if the string to be eval-ed did not compile (this mayhappen if open or close were imported with bad prototypes), orif Perl code executed during evaluation die()d. In these cases thevalue of $@ is the compile error, or the argument to die (whichwill interpolate $! and $?). (See also Fatal, though.)

Under a few operating systems, $^E may contain a more verbose errorindicator, such as in this case, "CDROM tray not closed." Systems thatdo not support extended error messages leave $^E the same as $!.

Finally, $? may be set to non-0 value if the external program/cdrom/install fails. The upper eight bits reflect specific errorconditions encountered by the program (the program's exit() value).The lower eight bits reflect mode of failure, like signal death andcore dump information. See wait(2) for details. In contrast to$! and $^E, which are set only if error condition is detected,the variable $? is set on each wait or pipe close,overwriting the old value. This is more like $@, which on everyeval() is always set on failure and cleared on success.

For more details, see the individual descriptions at $@, $!,$^E, and $?.

${^CHILD_ERROR_NATIVE}
The native status returned by the last pipe close, backtick (``)command, successful call to wait() or waitpid(), or from thesystem() operator. On POSIX-like systems this value can be decodedwith the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,WSTOPSIG and WIFCONTINUED functions provided by the POSIX module.
Under VMS this reflects the actual VMS exit status; i.e. it is thesame as $? when the pragma use vmsish 'status' is in effect.
This variable was added in Perl 5.8.9.
$EXTENDED_OS_ERROR
$^E
Error information specific to the current operating system. At themoment, this differs from $! under only VMS, OS/2, and Win32 (andfor MacPerl). On all other platforms, $^E is always just the sameas $!.
Under VMS, $^E provides the VMS status value from the last systemerror. This is more specific information about the last system errorthan that provided by $!. This is particularly important when $!is set to EVMSERR.
Under OS/2, $^E is set to the error code of the last call to OS/2API either via CRT, or directly from perl.
Under Win32, $^E always returns the last error information reportedby the Win32 call GetLastError() which describes the last errorfrom within the Win32 API. Most Win32-specific code will report errorsvia $^E. ANSI C and Unix-like calls set errno and so mostportable Perl code will report errors via $!.
Caveats mentioned in the description of $! generally apply to$^E, also.
This variable was added in Perl 5.003.
Mnemonic: Extra error explanation.
$EXCEPTIONS_BEING_CAUGHT
$^S
Current state of the interpreter.
```
$^S State
 ---------   -------------------
 undef   Parsing module/eval
 true (1) Executing an eval
 false (0)   Otherwise
 
```
The first state may happen in $SIG{__DIE__} and $SIG{__WARN__}handlers.
This variable was added in Perl 5.004.
$WARNING
$^W
The current value of the warning switch, initially true if -w wasused, false otherwise, but directly modifiable.
See also warnings.
Mnemonic: related to the -w switch.
${^WARNING_BITS}
The current set of warning checks enabled by the use warnings pragma.It has the same scoping as the $^H and %^H variables. The exactvalues are considered internal to the warnings pragma and may changebetween versions of Perl.
This variable was added in Perl 5.6.
$OS_ERROR
$ERRNO
$!
When referenced, $! retrieves the current valueof the C errno integer variable.If $! is assigned a numerical value, that value is stored in errno.When referenced as a string, $! yields the system error stringcorresponding to errno.
Many system or library calls set errno if they fail,to indicate the cause of failure. They usually do notset errno to zero if they succeed. This means errno,hence $!, is meaningful only immediately after a failure:
```
 if (open my $fh, "<", $filename) {
 # Here $! is meaningless.
 ...
  }
  else {
 # ONLY here is $! meaningful.
 ...
 # Already here $! might be meaningless.
  }
  # Since here we might have either success or failure,
  # $! is meaningless.
 
```
Here, meaningless means that $! may be unrelated to the outcomeof the open() operator. Assignment to $! is similarly ephemeral.It can be used immediately before invoking the die() operator,to set the exit value, or to inspect the system error stringcorresponding to error n, or to restore $! to a meaningful state.
Mnemonic: What just went bang?
%OS_ERROR
%ERRNO
%!
Each element of %! has a true value only if $! is set to thatvalue. For example, $!{ENOENT} is true if and only if the currentvalue of $! is ENOENT; that is, if the most recent error was "Nosuch file or directory" (or its moral equivalent: not all operatingsystems give that exact error, and certainly not all languages). Tocheck if a particular key is meaningful on your system, use exists$!{the_key}; for a list of legal keys, use keys %!. See Errnofor more information, and also see $!.
This variable was added in Perl 5.005.
$CHILD_ERROR
$?
The status returned by the last pipe close, backtick (``) command,successful call to wait() or waitpid(), or from the system()operator. This is just the 16-bit status word returned by thetraditional Unix wait() system call (or else is made up to looklike it). Thus, the exit value of the subprocess is really ($?>>8), and $? & 127 gives which signal, if any, the process diedfrom, and $? & 128 reports whether there was a core dump.
Additionally, if the h_errno variable is supported in C, its valueis returned via $? if any gethost*() function fails.
If you have installed a signal handler for SIGCHLD, thevalue of $? will usually be wrong outside that handler.
Inside an END subroutine $? contains the value that is going to begiven to exit(). You can modify $? in an END subroutine tochange the exit status of your program. For example:
```
 END {
 $? = 1 if $? == 255;  # die would make it 255
  }
 
```
Under VMS, the pragma use vmsish 'status' makes $? reflect theactual VMS exit status, instead of the default emulation of POSIXstatus; see $? in perlvms for details.
Mnemonic: similar to sh and ksh.
$EVAL_ERROR
$@
The Perl syntax error message from thelast eval() operator. If $@ isthe null string, the last eval() parsed and executed correctly(although the operations you invoked may have failed in the normalfashion).
Warning messages are not collected in this variable. You can, however,set up a routine to process warnings by setting $SIG{__WARN__} asdescribed in %SIG.
Mnemonic: Where was the syntax error "at"?

Variables related to the interpreter state

These variables provide information about the current interpreter state.

$COMPILING
$^C
The current value of the flag associated with the -c switch.Mainly of use with -MO=... to allow code to alter its behaviorwhen being compiled, such as for example to AUTOLOAD at compiletime rather than normal, deferred loading. Setting$^C = 1 is similar to calling B::minus_c.
This variable was added in Perl 5.6.
$DEBUGGING
$^D
The current value of the debugging flags. May be read or set. Like itscommand-line equivalent, you can use numeric or symbolic values, eg$^D = 10 or $^D = "st".
Mnemonic: value of -D switch.
${^ENCODING}
The object reference to the Encode object that is used to convertthe source code to Unicode. Thanks to this variable your Perl scriptdoes not have to be written in UTF-8. Default is undef. The directmanipulation of this variable is highly discouraged.
This variable was added in Perl 5.8.2.
${^GLOBAL_PHASE}
The current phase of the perl interpreter.
Possible values are:
- CONSTRUCT
  The PerlInterpreter* is being constructed via perl_construct. Thisvalue is mostly there for completeness and for use via theunderlying C variable PL_phase. It's not really possible for Perlcode to be executed unless construction of the interpreter isfinished.
- START
  This is the global compile-time. That includes, basically, everyBEGIN block executed directly or indirectly from during thecompile-time of the top-level program.
  This phase is not called "BEGIN" to avoid confusion withBEGIN-blocks, as those are executed during compile-time of anycompilation unit, not just the top-level program. A new, localisedcompile-time entered at run-time, for example by constructs aseval "use SomeModule" are not global interpreter phases, andtherefore aren't reflected by ${^GLOBAL_PHASE}.
- CHECK
  Execution of any CHECK blocks.
- INIT
  Similar to "CHECK", but for INIT-blocks, not CHECK blocks.
- RUN
  The main run-time, i.e. the execution of PL_main_root.
- END
  Execution of any END blocks.
- DESTRUCT
  Global destruction.
Also note that there's no value for UNITCHECK-blocks. That's becausethose are run for each compilation unit individually, and therefore isnot a global interpreter phase.
Not every program has to go through each of the possible phases, buttransition from one phase to another can only happen in the orderdescribed in the above list.
An example of all of the phases Perl code can see:
```
 BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
 
  INIT  { print "init-time: ${^GLOBAL_PHASE}\n" }
 
  CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
 
  {
  package Print::Phase;
 
  sub new {
  my ($class, $time) = @_;
  return bless \$time, $class;
  }
 
  sub DESTROY {
  my $self = shift;
  print "$$self: ${^GLOBAL_PHASE}\n";
  }
  }
 
  print "run-time: ${^GLOBAL_PHASE}\n";
 
  my $runtime = Print::Phase->new(
  "lexical variables are garbage collected before END"
  );
 
  END   { print "end-time: ${^GLOBAL_PHASE}\n" }
 
  our $destruct = Print::Phase->new(
  "package variables are garbage collected after END"
  );
 
```
This will print out
```
 compile-time: START
  check-time: CHECK
  init-time: INIT
  run-time: RUN
  lexical variables are garbage collected before END: RUN
  end-time: END
  package variables are garbage collected after END: DESTRUCT
 
```
This variable was added in Perl 5.14.0.
$^H
WARNING: This variable is strictly forinternal use only. Its availability,behavior, and contents are subject to change without notice.
This variable contains compile-time hints for the Perl interpreter. At theend of compilation of a BLOCK the value of this variable is restored to thevalue when the interpreter started to compile the BLOCK.
When perl begins to parse any block construct that provides a lexical scope(e.g., eval body, required file, subroutine body, loop body, or conditionalblock), the existing value of $^H is saved, but its value is left unchanged.When the compilation of the block is completed, it regains the saved value.Between the points where its value is saved and restored, code thatexecutes within BEGIN blocks is free to change the value of $^H.
This behavior provides the semantic of lexical scoping, and is used in,for instance, the use strict pragma.
The contents should be an integer; different bits of it are used fordifferent pragmatic flags. Here's an example:
```
 sub add_100 { $^H |= 0x100 }
 
  sub foo {
 BEGIN { add_100() }
 bar->baz($boon);
  }
 
```
Consider what happens during execution of the BEGIN block. At this pointthe BEGIN block has already been compiled, but the body of foo() is stillbeing compiled. The new value of $^Hwill therefore be visible only whilethe body of foo() is being compiled.
Substitution of BEGIN { add_100() } block with:
```
 BEGIN { require strict; strict->import('vars') }
 
```
demonstrates how use strict 'vars' is implemented. Here's a conditionalversion of the same lexical pragma:
```
 BEGIN {
 require strict; strict->import('vars') if $condition
  }
 
```
This variable was added in Perl 5.003.
%^H
The %^H hash provides the same scoping semantic as $^H. This makesit useful for implementation of lexically scoped pragmas. Seeperlpragma.
When putting items into %^H, in order to avoid conflicting with otherusers of the hash there is a convention regarding which keys to use.A module should use only keys that begin with the module's name (thename of its main package) and a "/" character. For example, a moduleFoo::Bar should use keys such as Foo::Bar/baz.
This variable was added in Perl 5.6.
${^OPEN}
An internal variable used by PerlIO. A string in two parts, separatedby a \0 byte, the first part describes the input layers, the secondpart describes the output layers.
This variable was added in Perl 5.8.0.
$PERLDB
$^P
The internal variable for debugging support. The meanings of thevarious bits are subject to change, but currently indicate:
0
x01
Debug subroutine enter/exit.
0
x02
Line-by-line debugging. Causes DB::DB() subroutine to be called foreach statement executed. Also causes saving source code lines (like0x400).
0
x04
Switch off optimizations.
0
x08
Preserve more data for future interactive inspections.
0
x10
Keep info about source lines on which a subroutine is defined.
0
x20
Start with single-step on.
0
x40
Use subroutine address instead of name when reporting.
0
x80
Report goto &subroutine as well.
0
x100
Provide informative "file" names for evals based on the place they were compiled.
0
x200
Provide informative names to anonymous subroutines based on the place theywere compiled.
0
x400
Save source code lines into @{"_<$filename"}.
Some bits may be relevant at compile-time only, some atrun-time only. This is a new mechanism and the details may change.See also perldebguts.
${^TAINT}
Reflects if taint mode is on or off. 1 for on (the program was run with-T), 0 for off, -1 when only taint warnings are enabled (i.e. with-t or -TU).
This variable is read-only.
This variable was added in Perl 5.8.
${^UNICODE}
Reflects certain Unicode settings of Perl. See perlrundocumentation for the -C switch for more information aboutthe possible values.
This variable is set during Perl startup and is thereafter read-only.
This variable was added in Perl 5.8.2.
${^UTF8CACHE}
This variable controls the state of the internal UTF-8 offset caching code.1 for on (the default), 0 for off, -1 to debug the caching code by checkingall its results against linear scans, and panicking on any discrepancy.
This variable was added in Perl 5.8.9.
${^UTF8LOCALE}
This variable indicates whether a UTF-8 locale was detected by perl atstartup. This information is used by perl when it's inadjust-utf8ness-to-locale mode (as when run with the -CL command-lineswitch); see perlrun for more info on this.
This variable was added in Perl 5.8.8.

Deprecated and removed variables

Deprecating a variable announces the intent of the perl maintainers toeventually remove the variable from the language. It may still beavailable despite its status. Using a deprecated variable triggersa warning.

Once a variable is removed, its use triggers an error telling youthe variable is unsupported.

See perldiag for details about error messages.

$OFMT
$#
$# was a variable that could be used to format printed numbers.After a deprecation cycle, its magic was removed in Perl 5.10 andusing it now triggers a warning: $# is no longer supported.
This is not the sigil you use in front of an array name to get thelast index, like $#array. That's still how you get the last indexof an array in Perl. The two have nothing to do with each other.
Deprecated in Perl 5.
Removed in Perl 5.10.
$*
$* was a variable that you could use to enable multiline matching.After a deprecation cycle, its magic was removed in Perl 5.10.Using it now triggers a warning: $* is no longer supported.You should use the /s and /m regexp modifiers instead.
Deprecated in Perl 5.
Removed in Perl 5.10.
$ARRAY_BASE
$[
This variable stores the index of the first element in an array, andof the first character in a substring. The default is 0, but you couldtheoretically set it to 1 to make Perl behave more like awk (or Fortran)when subscripting and when evaluating the index() and substr() functions.
As of release 5 of Perl, assignment to $[ is treated as a compilerdirective, and cannot influence the behavior of any other file.(That's why you can only assign compile-time constants to it.)Its use is highly discouraged.
Prior to Perl 5.10, assignment to $[ could be seen from outer lexicalscopes in the same file, unlike other compile-time directives (such asstrict). Using local() on it would bind its value strictly to a lexicalblock. Now it is always lexically scoped.
As of Perl 5.16, it is implemented by the arybase module. Seearybase for more details on its behaviour.
Under use v5.16, or no feature "array_base", $[ no longer has anyeffect, and always contains 0. Assigning 0 to it is permitted, but anyother value will produce an error.
Mnemonic: [ begins subscripts.
Deprecated in Perl 5.12.
$OLD_PERL_VERSION
$]
See $^V for a more modern representation of the Perl version that allowsaccurate string comparisons.
The version + patchlevel / 1000 of the Perl interpreter. This variablecan be used to determine whether the Perl interpreter executing ascript is in the right range of versions:
```
 warn "No checksumming!\n" if $] < 3.019;
 
```
The floating point representation can sometimes lead to inaccuratenumeric comparisons.
See also the documentation of use VERSION and require VERSIONfor a convenient way to fail if the running Perl interpreter is too old.
Mnemonic: Is this version of perl in the right bracket?

Page index

NAME
DESCRIPTION
- The Syntax of Variable Names
SPECIAL VARIABLES

Source : perldoc.perl.org - Official documentation for the Perl programming language
Site maintained by Jon Allen (JJ) See the project page for more details
Documentation maintained by the Perl 5 Porters

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