perl[ -sTtuUWX ][ -hv ] [ -V[:configvar] ][ -cw ] [ -d[t][:debugger] ] [ -D[number/list] ][ -pna ] [ -Fpattern ] [ -l[octal] ] [ -0[octal/hexadecimal] ][ -Idir ] [ -m[-]module ] [ -M[-]'module...' ] [ -f ][ -C [number/list] ][ -S ][ -x[dir] ][ -i[extension] ][ [-e|-E] 'command' ] [ -- ] [ programfile ] [ argument ]...

DESCRIPTION

The normal way to run a Perl program is by making it directlyexecutable, or else by passing the name of the source file as anargument on the command line. (An interactive Perl environmentis also possible--see perldebug for details on how to do that.)Upon startup, Perl looks for your program in one of the followingplaces:

1.: Specified line by line via -e or -E switches on the command line.
2.: Contained in the file specified by the first filename on the command line.(Note that systems supporting the #! notation invoke interpreters thisway. See Location of Perl.)
3.: Passed in implicitly via standard input. This works only if there areno filename arguments--to pass arguments to a STDIN-read program youmust explicitly specify a "-" for the program name.

With methods 2 and 3, Perl starts parsing the input file from thebeginning, unless you've specified a -x switch, in which case itscans for the first line starting with #! and containing the word"perl", and starts there instead. This is useful for running a programembedded in a larger message. (In this case you would indicate the endof the program using the __END__ token.)

The #! line is always examined for switches as the line is beingparsed. Thus, if you're on a machine that allows only one argumentwith the #! line, or worse, doesn't even recognize the #! line, youstill can get consistent switch behaviour regardless of how Perl wasinvoked, even if -x was used to find the beginning of the program.

Because historically some operating systems silently chopped offkernel interpretation of the #! line after 32 characters, someswitches may be passed in on the command line, and some may not;you could even get a "-" without its letter, if you're not careful.You probably want to make sure that all your switches fall eitherbefore or after that 32-character boundary. Most switches don'tactually care if they're processed redundantly, but getting a "-"instead of a complete switch could cause Perl to try to executestandard input instead of your program. And a partial -I switchcould also cause odd results.

Some switches do care if they are processed twice, for instancecombinations of -l and -0. Either put all the switches afterthe 32-character boundary (if applicable), or replace the use of-0digits by BEGIN{ $/ = "\0digits"; }.

Parsing of the #! switches starts wherever "perl" is mentioned in the line.The sequences "-*" and "- " are specifically ignored so that you could,if you were so inclined, say

 #!/bin/sh
  #! -*-perl-*-
  eval 'exec perl -x -wS $0 ${1+"$@"}'
  if 0;

to let Perl see the -p switch.

A similar trick involves the env program, if you have it.

 #!/usr/bin/env perl

The examples above use a relative path to the perl interpreter,getting whatever version is first in the user's path. If you wanta specific version of Perl, say, perl5.005_57, you should placethat directly in the #! line's path.

If the #! line does not contain the word "perl" nor the word "indir"the program named after the #! is executed instead of the Perlinterpreter. This is slightly bizarre, but it helps people on machinesthat don't do #!, because they can tell a program that their SHELL is/usr/bin/perl, and Perl will then dispatch the program to the correctinterpreter for them.

After locating your program, Perl compiles the entire program to aninternal form. If there are any compilation errors, execution of theprogram is not attempted. (This is unlike the typical shell script,which might run part-way through before finding a syntax error.)

If the program is syntactically correct, it is executed. If the programruns off the end without hitting an exit() or die() operator, an implicitexit(0) is provided to indicate successful completion.

#! and quoting on non-Unix systems

Unix's #! technique can be simulated on other systems:

OS/2
Put
```
 extproc perl -S -your_switches
 
```
as the first line in *.cmd file (-S due to a bug in cmd.exe's`extproc' handling).
MS-DOS
Create a batch file to run your program, and codify it inALTERNATE_SHEBANG (see the dosish.h file in the sourcedistribution for more information).
Win95/NT
The Win95/NT installation, when using the ActiveState installer for Perl,will modify the Registry to associate the .pl extension with the perlinterpreter. If you install Perl by other means (including building fromthe sources), you may have to modify the Registry yourself. Note thatthis means you can no longer tell the difference between an executablePerl program and a Perl library file.
VMS
Put
```
 $ perl -mysw 'f$env("procedure")' 'p1' 'p2' 'p3' 'p4' 'p5' 'p6' 'p7' 'p8' !
  $ exit++ + ++$status != 0 and $exit = $status = undef;
 
```
at the top of your program, where -mysw are any command line switches youwant to pass to Perl. You can now invoke the program directly, by sayingperl program, or as a DCL procedure, by saying @program (or implicitlyvia DCL$PATH by just using the name of the program).
This incantation is a bit much to remember, but Perl will display it foryou if you say perl "-V:startperl".

Command-interpreters on non-Unix systems have rather different ideason quoting than Unix shells. You'll need to learn the specialcharacters in your command-interpreter (*, \ and " arecommon) and how to protect whitespace and these characters to runone-liners (see -e below).

On some systems, you may have to change single-quotes to double ones,which you must not do on Unix or Plan 9 systems. You might alsohave to change a single % to a %%.

For example:

 # Unix
  perl -e 'print "Hello world\n"'
 
  # MS-DOS, etc.
  perl -e "print \"Hello world\n\""
 
  # VMS
  perl -e "print ""Hello world\n"""

The problem is that none of this is reliable: it depends on thecommand and it is entirely possible neither works. If 4DOS werethe command shell, this would probably work better:

 perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""

CMD.EXE in Windows NT slipped a lot of standard Unix functionality inwhen nobody was looking, but just try to find documentation for itsquoting rules.

There is no general solution to all of this. It's just a mess.

Location of Perl

It may seem obvious to say, but Perl is useful only when users caneasily find it. When possible, it's good for both /usr/bin/perland /usr/local/bin/perl to be symlinks to the actual binary. Ifthat can't be done, system administrators are strongly encouragedto put (symlinks to) perl and its accompanying utilities into adirectory typically found along a user's PATH, or in some otherobvious and convenient place.

In this documentation, #!/usr/bin/perl on the first line of the programwill stand in for whatever method works on your system. You areadvised to use a specific path if you care about a specific version.

 #!/usr/local/bin/perl5.00554

or if you just want to be running at least version, place a statementlike this at the top of your program:

 use 5.005_54;

Command Switches

As with all standard commands, a single-character switch may beclustered with the following switch, if any.

 #!/usr/bin/perl -spi.orig# same as -s -p -i.orig

Switches include:

-0[octal/hexadecimal]
specifies the input record separator ($/) as an octal orhexadecimal number. If there are no digits, the null character is theseparator. Other switches may precede or follow the digits. Forexample, if you have a version of find which can print filenamesterminated by the null character, you can say this:
```
 find . -name '*.orig' -print0 | perl -n0e unlink
 
```
The special value 00 will cause Perl to slurp files in paragraph mode.Any value 0400 or above will cause Perl to slurp files whole, but by conventionthe value 0777 is the one normally used for this purpose.
You can also specify the separator character using hexadecimal notation:-0xHHH..., where the H are valid hexadecimal digits. Unlikethe octal form, this one may be used to specify any Unicode character, eventhose beyond 0xFF. So if you really want a record separator of 0777,specify it as -0x1FF. (This means that you cannot use the -x optionwith a directory name that consists of hexadecimal digits, or else Perlwill think you have specified a hex number to -0.)
-a
turns on autosplit mode when used with a -n or -p. An implicitsplit command to the @F array is done as the first thing inside theimplicit while loop produced by the -n or -p.
```
 perl -ane 'print pop(@F), "\n"'
 
```
is equivalent to
```
 while (<>) {
 @F = split(' ');
 print pop(@F), "\n";
  }
 
```
An alternate delimiter may be specified using -F.
-C [number/list]
The -C flag controls some of the Perl Unicode features.
As of 5.8.1, the -C can be followed either by a number or a listof option letters. The letters, their numeric values, and effectsare as follows; listing the letters is equal to summing the numbers.
```
 I 1   STDIN is assumed to be in UTF-8
  O 2   STDOUT will be in UTF-8
  E 4   STDERR will be in UTF-8
  S 7   I + O + E
  i 8   UTF-8 is the default PerlIO layer for input streams
  o 16   UTF-8 is the default PerlIO layer for output streams
  D 24   i + o
  A 32   the @ARGV elements are expected to be strings encoded
   in UTF-8
  L 64   normally the "IOEioA" are unconditional, the L makes
   them conditional on the locale environment variables
   (the LC_ALL, LC_TYPE, and LANG, in the order of
   decreasing precedence) -- if the variables indicate
   UTF-8, then the selected "IOEioA" are in effect
  a   256   Set ${^UTF8CACHE} to -1, to run the UTF-8 caching
   code in debugging mode.
 
```
For example, -COE and -C6 will both turn on UTF-8-ness on bothSTDOUT and STDERR. Repeating letters is just redundant, not cumulativenor toggling.
The io options mean that any subsequent open() (or similar I/Ooperations) in the current file scope will have the :utf8 PerlIO layerimplicitly applied to them, in other words, UTF-8 is expected from anyinput stream, and UTF-8 is produced to any output stream. This is justthe default, with explicit layers in open() and with binmode() one canmanipulate streams as usual.
-C on its own (not followed by any number or option list), or theempty string "" for the PERL_UNICODE environment variable, has thesame effect as -CSDL. In other words, the standard I/O handles andthe default open() layer are UTF-8-fied but only if the localeenvironment variables indicate a UTF-8 locale. This behaviour followsthe implicit (and problematic) UTF-8 behaviour of Perl 5.8.0.(See UTF-8 no longer default under UTF-8 locales in perl581delta.)
You can use -C0 (or "0" for PERL_UNICODE) to explicitlydisable all the above Unicode features.
The read-only magic variable ${^UNICODE} reflects the numeric valueof this setting. This variable is set during Perl startup and isthereafter read-only. If you want runtime effects, use the three-argopen() (see open), the two-arg binmode() (see binmode),and the open pragma (see open).
(In Perls earlier than 5.8.1 the -C switch was a Win32-only switchthat enabled the use of Unicode-aware "wide system call" Win32 APIs.This feature was practically unused, however, and the command lineswitch was therefore "recycled".)
Note: Since perl 5.10.1, if the -C option is used on the #! line,it must be specified on the command line as well, since the standard streamsare already set up at this point in the execution of the perl interpreter.You can also use binmode() to set the encoding of an I/O stream.
-c
causes Perl to check the syntax of the program and then exit withoutexecuting it. Actually, it will execute and BEGIN, UNITCHECK,or CHECK blocks and any use statements: these are considered asoccurring outside the execution of your program. INIT and ENDblocks, however, will be skipped.
-d
-dt
runs the program under the Perl debugger. See perldebug.If t is specified, it indicates to the debugger that threadswill be used in the code being debugged.
-d:MOD[=bar,baz]
-dt:MOD[=bar,baz]
runs the program under the control of a debugging, profiling, or tracingmodule installed as Devel::MOD. E.g., -d:DProf executes theprogram using the Devel::DProf profiler. As with the -M flag, optionsmay be passed to the Devel::MOD package where they will be receivedand interpreted by the Devel::MOD::import routine. Again, like -M,use --d:-MOD to call Devel::MOD::unimport instead of import. Thecomma-separated list of options must follow a = character. If t isspecified, it indicates to the debugger that threads will be used in thecode being debugged. See perldebug.
-Dletters

-Dnumber

sets debugging flags. To watch how it executes your program, use-Dtls. (This works only if debugging is compiled into yourPerl.) Another nice value is -Dx, which lists your compiledsyntax tree. And -Dr displays compiled regular expressions;the format of the output is explained in perldebguts.

As an alternative, specify a number instead of list of letters (e.g.,-D14 is equivalent to -Dtls):

 1  p  Tokenizing and parsing (with v, displays parse stack)
  2  s  Stack snapshots (with v, displays all stacks)
   4  l  Context (loop) stack processing
   8  t  Trace execution
       16  o  Method and overloading resolution
       32  c  String/numeric conversions
       64  P  Print profiling info, source file input state
     128  m  Memory and SV allocation
   256  f  Format processing
   512  r  Regular expression parsing and execution
  1024  x  Syntax tree dump
  2048  u  Tainting checks
  4096  U  Unofficial, User hacking (reserved for private,
   unreleased use)
  8192  H  Hash dump -- usurps values()
  16384  X  Scratchpad allocation
  32768  D  Cleaning up
    131072  T  Tokenizing
    262144  R  Include reference counts of dumped variables (eg when
   using -Ds)
    524288  J  show s,t,P-debug (don't Jump over) on opcodes within
     package DB
     1048576  v  Verbose: use in conjunction with other flags
     2097152  C  Copy On Write
     4194304  A  Consistency checks on internal structures
     8388608  q  quiet - currently only suppresses the "EXECUTING"
     message
   16777216  M  trace smart match resolution
   33554432  B  dump suBroutine definitions, including special Blocks
   like BEGIN

All these flags require -DDEBUGGING when you compile the Perlexecutable (but see :opd in Devel::Peek or 'debug' mode in rewhich may change this).See the INSTALL file in the Perl source distributionfor how to do this. This flag is automatically set if you include -goption when Configure asks you about optimizer/debugger flags.

If you're just trying to get a print out of each line of Perl codeas it executes, the way that sh -x provides for shell scripts,you can't use Perl's -D switch. Instead do this

  # If you have "env" utility
   env PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
 
   # Bourne shell syntax
   $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
 
   # csh syntax
   % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)

See perldebug for details and variations.

-e commandline
may be used to enter one line of program. If -e is given, Perlwill not look for a filename in the argument list. Multiple -ecommands may be given to build up a multi-line script. Make sureto use semicolons where you would in a normal program.
-E commandline
behaves just like -e, except that it implicitly enables alloptional features (in the main compilation unit). See feature.
-f
Disable executing $Config{sitelib}/sitecustomize.pl at startup.
Perl can be built so that it by default will try to execute$Config{sitelib}/sitecustomize.pl at startup (in a BEGIN block).This is a hook that allows the sysadmin to customize how Perl behaves.It can for instance be used to add entries to the @INC array to make Perlfind modules in non-standard locations.
Perl actually inserts the following code:
```
 BEGIN {
  do { local $!; -f "$Config{sitelib}/sitecustomize.pl"; }
  && do "$Config{sitelib}/sitecustomize.pl";
  }
 
```
Since it is an actual do (not a require), sitecustomize.pldoesn't need to return a true value. The code is run in package main,in its own lexical scope. However, if the script dies, $@ will notbe set.
The value of $Config{sitelib} is also determined in C code and notread from Config.pm, which is not loaded.
The code is executed very early. For example, any changes made to@INC will show up in the output of `perl -V`. Of course, ENDblocks will be likewise executed very late.
To determine at runtime if this capability has been compiled in yourperl, you can check the value of $Config{usesitecustomize}.
-Fpattern
specifies the pattern to split on if -a is also in effect. Thepattern may be surrounded by //, "", or '', otherwise it will beput in single quotes. You can't use literal whitespace in the pattern.
-h
prints a summary of the options.
-i[extension]
specifies that files processed by the <> construct are to beedited in-place. It does this by renaming the input file, opening theoutput file by the original name, and selecting that output file as thedefault for print() statements. The extension, if supplied, is used tomodify the name of the old file to make a backup copy, following theserules:
If no extension is supplied, no backup is made and the current file isoverwritten.
If the extension doesn't contain a *, then it is appended to theend of the current filename as a suffix. If the extension doescontain one or more * characters, then each * is replacedwith the current filename. In Perl terms, you could think of thisas:
```
 ($backup = $extension) =~ s/\*/$file_name/g;
 
```
This allows you to add a prefix to the backup file, instead of (or inaddition to) a suffix:
```
 $ perl -pi'orig_*' -e 's/bar/baz/' fileA  # backup to
  # 'orig_fileA'
 
```
Or even to place backup copies of the original files into anotherdirectory (provided the directory already exists):
```
 $ perl -pi'old/*.orig' -e 's/bar/baz/' fileA  # backup to
  # 'old/fileA.orig'
 
```
These sets of one-liners are equivalent:
```
 $ perl -pi -e 's/bar/baz/' fileA  # overwrite current file
  $ perl -pi'*' -e 's/bar/baz/' fileA   # overwrite current file
 
  $ perl -pi'.orig' -e 's/bar/baz/' fileA   # backup to 'fileA.orig'
  $ perl -pi'*.orig' -e 's/bar/baz/' fileA  # backup to 'fileA.orig'
 
```
From the shell, saying
```
 $ perl -p -i.orig -e "s/foo/bar/; ... "
 
```
is the same as using the program:
```
 #!/usr/bin/perl -pi.orig
  s/foo/bar/;
 
```
which is equivalent to
```
 #!/usr/bin/perl
  $extension = '.orig';
  LINE: while (<>) {
 if ($ARGV ne $oldargv) {
  if ($extension !~ /\*/) {
 $backup = $ARGV . $extension;
  }
  else {
 ($backup = $extension) =~ s/\*/$ARGV/g;
  }
  rename($ARGV, $backup);
  open(ARGVOUT, ">$ARGV");
  select(ARGVOUT);
  $oldargv = $ARGV;
 }
 s/foo/bar/;
  }
  continue {
 print;# this prints to original filename
  }
  select(STDOUT);
 
```
except that the -i form doesn't need to compare $ARGV to $oldargv toknow when the filename has changed. It does, however, use ARGVOUT forthe selected filehandle. Note that STDOUT is restored as the defaultoutput filehandle after the loop.
As shown above, Perl creates the backup file whether or not any outputis actually changed. So this is just a fancy way to copy files:
```
 $ perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...
 or
  $ perl -p -i'.orig' -e 1 file1 file2 file3...
 
```
You can use eof without parentheses to locate the end of each inputfile, in case you want to append to each file, or reset line numbering(see example in eof).
If, for a given file, Perl is unable to create the backup file asspecified in the extension then it will skip that file and continue onwith the next one (if it exists).
For a discussion of issues surrounding file permissions and -i,see Why does Perl let me delete read-only files? Why does -i clobber protected files? Isn't this a bug in Perl? in perlfaq5.
You cannot use -i to create directories or to strip extensions fromfiles.
Perl does not expand ~ in filenames, which is good, since somefolks use it for their backup files:
```
 $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...
 
```
Note that because -i renames or deletes the original file beforecreating a new file of the same name, Unix-style soft and hard links willnot be preserved.
Finally, the -i switch does not impede execution when nofiles are given on the command line. In this case, no backup is made(the original file cannot, of course, be determined) and processingproceeds from STDIN to STDOUT as might be expected.
-Idirectory
Directories specified by -I are prepended to the search path formodules (@INC).
-l[octnum]
enables automatic line-ending processing. It has two separateeffects. First, it automatically chomps $/ (the input recordseparator) when used with -n or -p. Second, it assigns $\(the output record separator) to have the value of octnum sothat any print statements will have that separator added back on.If octnum is omitted, sets $\ to the current value of$/. For instance, to trim lines to 80 columns:
```
 perl -lpe 'substr($_, 80) = ""'
 
```
Note that the assignment $\ = $/ is done when the switch is processed,so the input record separator can be different than the output recordseparator if the -l switch is followed by a -0 switch:
```
 gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
 
```
This sets $\ to newline and then sets $/ to the null character.
-m[-]module
-M[-]module
-M[-]'module ...'
-[mM][-]module=arg[,arg]...
-mmodule executes use module (); before executing yourprogram.
-Mmodule executes use module ; before executing yourprogram. You can use quotes to add extra code after the module name,e.g., '-MMODULE qw(foo bar)'.
If the first character after the -M or -m is a dash (-)then the 'use' is replaced with 'no'.
A little builtin syntactic sugar means you can also say-mMODULE=foo,bar or -MMODULE=foo,bar as a shortcut for'-MMODULE qw(foo bar)'. This avoids the need to use quotes whenimporting symbols. The actual code generated by -MMODULE=foo,bar isuse module split(/,/,q{foo,bar}). Note that the = formremoves the distinction between -m and -M.
A consequence of this is that -MMODULE=number never does a version check,unless MODULE::import() itself is set up to do a version check, whichcould happen for example if MODULE inherits from Exporter.
-n
causes Perl to assume the following loop around your program, whichmakes it iterate over filename arguments somewhat like sed -n orawk:
```
  LINE:
  while (<>) {
 ...# your program goes here
  }
 
```
Note that the lines are not printed by default. See -p to havelines printed. If a file named by an argument cannot be opened forsome reason, Perl warns you about it and moves on to the next file.
Also note that <> passes command line arguments toopen, which doesn't necessarily interpret them as file names.See perlop for possible security implications.
Here is an efficient way to delete all files that haven't been modified forat least a week:
```
 find . -mtime +7 -print | perl -nle unlink
 
```
This is faster than using the -exec switch of find because you don'thave to start a process on every filename found. It does suffer fromthe bug of mishandling newlines in pathnames, which you can fix ifyou follow the example under -0.
BEGIN and END blocks may be used to capture control before or afterthe implicit program loop, just as in awk.
-p
causes Perl to assume the following loop around your program, whichmakes it iterate over filename arguments somewhat like sed:
```
  LINE:
  while (<>) {
 ...# your program goes here
  } continue {
 print or die "-p destination: $!\n";
  }
 
```
If a file named by an argument cannot be opened for some reason, Perlwarns you about it, and moves on to the next file. Note that thelines are printed automatically. An error occurring during printing istreated as fatal. To suppress printing use the -n switch. A -poverrides a -n switch.
BEGIN and END blocks may be used to capture control before or afterthe implicit loop, just as in awk.
-s
enables rudimentary switch parsing for switches on the commandline after the program name but before any filename arguments (or beforean argument of --). Any switch found there is removed from @ARGV and sets thecorresponding variable in the Perl program. The following programprints "1" if the program is invoked with a -xyz switch, and "abc"if it is invoked with -xyz=abc.
```
 #!/usr/bin/perl -s
  if ($xyz) { print "$xyz\n" }
 
```
Do note that a switch like --help creates the variable ${-help}, which is not compliantwith use strict "refs". Also, when using this option on a script withwarnings enabled you may get a lot of spurious "used only once" warnings.
-S
makes Perl use the PATH environment variable to search for theprogram unless the name of the program contains path separators.
On some platforms, this also makes Perl append suffixes to thefilename while searching for it. For example, on Win32 platforms,the ".bat" and ".cmd" suffixes are appended if a lookup for theoriginal name fails, and if the name does not already end in oneof those suffixes. If your Perl was compiled with DEBUGGING turnedon, using the -Dp switch to Perl shows how the search progresses.
Typically this is used to emulate #! startup on platforms that don'tsupport #!. It's also convenient when debugging a script that uses #!,and is thus normally found by the shell's $PATH search mechanism.
This example works on many platforms that have a shell compatible withBourne shell:
```
 #!/usr/bin/perl
  eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
  if $running_under_some_shell;
 
```
The system ignores the first line and feeds the program to /bin/sh,which proceeds to try to execute the Perl program as a shell script.The shell executes the second line as a normal shell command, and thusstarts up the Perl interpreter. On some systems $0 doesn't alwayscontain the full pathname, so the -S tells Perl to search for theprogram if necessary. After Perl locates the program, it parses thelines and ignores them because the variable $running_under_some_shellis never true. If the program will be interpreted by csh, you will needto replace ${1+"$@"} with $*, even though that doesn't understandembedded spaces (and such) in the argument list. To start up sh ratherthan csh, some systems may have to replace the #! line with a linecontaining just a colon, which will be politely ignored by Perl. Othersystems can't control that, and need a totally devious construct thatwill work under any of csh, sh, or Perl, such as the following:
```
eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
 & eval 'exec /usr/bin/perl -wS $0 $argv:q'
 if $running_under_some_shell;
 
```
If the filename supplied contains directory separators (and so is anabsolute or relative pathname), and if that file is not found,platforms that append file extensions will do so and try to lookfor the file with those extensions added, one by one.
On DOS-like platforms, if the program does not contain directoryseparators, it will first be searched for in the current directorybefore being searched for on the PATH. On Unix platforms, theprogram will be searched for strictly on the PATH.
-t
Like -T, but taint checks will issue warnings rather than fatalerrors. These warnings can now be controlled normally with no warningsqw(taint).
Note: This is not a substitute for -T! This is meant to beused only as a temporary development aid while securing legacy code:for real production code and for new secure code written from scratch,always use the real -T.
-T
turns on "taint" so you can test them. Ordinarilythese checks are done only when running setuid or setgid. It's agood idea to turn them on explicitly for programs that run on behalfof someone else whom you might not necessarily trust, such as CGIprograms or any internet servers you might write in Perl. Seeperlsec for details. For security reasons, this option must beseen by Perl quite early; usually this means it must appear earlyon the command line or in the #! line for systems which supportthat construct.
-u
This switch causes Perl to dump core after compiling yourprogram. You can then in theory take this core dump and turn itinto an executable file by using the undump program (not supplied).This speeds startup at the expense of some disk space (which youcan minimize by stripping the executable). (Still, a "hello world"executable comes out to about 200K on my machine.) If you want toexecute a portion of your program before dumping, use the dump()operator instead. Note: availability of undump is platformspecific and may not be available for a specific port of Perl.
-U
allows Perl to do unsafe operations. Currently the only "unsafe"operations are attempting to unlink directories while running as superuserand running setuid programs with fatal taint checks turned into warnings.Note that warnings must be enabled along with this option to actuallygenerate the taint-check warnings.
-v
prints the version and patchlevel of your perl executable.
-V
prints summary of the major perl configuration values and the currentvalues of @INC.
-V:configvar
Prints to STDOUT the value of the named configuration variable(s),with multiples when your configvar argument looks like a regex (hasnon-letters). For example:
```
 $ perl -V:libc
 libc='/lib/libc-2.2.4.so';
  $ perl -V:lib.
 libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
 libc='/lib/libc-2.2.4.so';
  $ perl -V:lib.*
 libpth='/usr/local/lib /lib /usr/lib';
 libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
 lib_ext='.a';
 libc='/lib/libc-2.2.4.so';
 libperl='libperl.a';
 ....
 
```
Additionally, extra colons can be used to control formatting. Atrailing colon suppresses the linefeed and terminator ";", allowingyou to embed queries into shell commands. (mnemonic: PATH separator":".)
```
 $ echo "compression-vars: " `perl -V:z.*: ` " are here !"
  compression-vars:  zcat='' zip='zip'  are here !
 
```
A leading colon removes the "name=" part of the response, this allowsyou to map to the name you need. (mnemonic: empty label)
```
 $ echo "goodvfork="`./perl -Ilib -V::usevfork`
  goodvfork=false;
 
```
Leading and trailing colons can be used together if you needpositional parameter values without the names. Note that in the casebelow, the PERL_API params are returned in alphabetical order.
```
 $ echo building_on `perl -V::osname: -V::PERL_API_.*:` now
  building_on 'linux' '5' '1' '9' now
 
```
-w
prints warnings about dubious constructs, such as variable namesmentioned only once and scalar variables usedbefore being set; redefined subroutines; references to undefinedfilehandles; filehandles opened read-only that you are attemptingto write on; values used as a number that don't look like numbers;using an array as though it were a scalar; if your subroutinesrecurse more than 100 deep; and innumerable other things.
This switch really just enables the global $^W variable; normally,the lexically scoped use warnings pragma is preferred. Youcan disable or promote into fatal errors specific warnings using__WARN__ hooks, as described in perlvar and warn.See also perldiag and perltrap. A fine-grained warningfacility is also available if you want to manipulate entire classesof warnings; see warnings or perllexwarn.
-W
Enables all warnings regardless of no warnings or $^W.See perllexwarn.
-X
Disables all warnings regardless of use warnings or $^W.See perllexwarn.
-x
-xdirectory
tells Perl that the program is embedded in a larger chunk of unrelatedtext, such as in a mail message. Leading garbage will bediscarded until the first line that starts with #! and contains thestring "perl". Any meaningful switches on that line will be applied.
All references to line numbers by the program (warnings, errors, ...)will treat the #! line as the first line.Thus a warning on the 2nd line of the program, which is on the 100thline in the file will be reported as line 2, not as line 100.This can be overridden by using the #line directive.(See Plain Old Comments (Not!) in perlsyn)
If a directory name is specified, Perl will switch to that directorybefore running the program. The -x switch controls only thedisposal of leading garbage. The program must be terminated with__END__ if there is trailing garbage to be ignored; the programcan process any or all of the trailing garbage via the DATA filehandleif desired.
The directory, if specified, must appear immediately following the -xwith no intervening whitespace.

ENVIRONMENT

HOME
Used if chdir has no argument.
LOGDIR
Used if chdir has no argument and HOME is not set.
PATH
Used in executing subprocesses, and in finding the program if -S isused.
PERL5LIB
A list of directories in which to look for Perl libraryfiles before looking in the standard library and the currentdirectory. Any architecture-specific directories under the specifiedlocations are automatically included if they exist, with this lookupdone at interpreter startup time.
If PERL5LIB is not defined, PERLLIB is used. Directories are separated(like in PATH) by a colon on Unixish platforms and by a semicolon onWindows (the proper path separator being given by the command perl-V:path_sep).
When running taint checks, either because the program was running setuid orsetgid, or the -T or -t switch was specified, neither PERL5LIB norPERLLIB is consulted. The program should instead say:
```
 use lib "/my/directory";
 
```
PERL5OPT
Command-line options (switches). Switches in this variable are treatedas if they were on every Perl command line. Only the -[CDIMUdmtwW]switches are allowed. When running taint checks (either because theprogram was running setuid or setgid, or because the -T or -tswitch was used), this variable is ignored. If PERL5OPT begins with- T, tainting will be enabled and subsequent options ignored. IfPERL5OPT begins with -t, tainting will be enabled, a writable dotremoved from @INC, and subsequent options honored.
PERLIO
A space (or colon) separated list of PerlIO layers. If perl is builtto use PerlIO system for IO (the default) these layers affect Perl's IO.
It is conventional to start layer names with a colon (for example, :perlio) toemphasize their similarity to variable "attributes". But the code that parseslayer specification strings, which is also used to decode the PERLIOenvironment variable, treats the colon as a separator.
An unset or empty PERLIO is equivalent to the default set of layers foryour platform; for example, :unix:perlio on Unix-like systemsand :unix:crlf on Windows and other DOS-like systems.
The list becomes the default for all Perl's IO. Consequently only built-inlayers can appear in this list, as external layers (such as :encoding()) needIO in order to load them!. See open pragma for how to add externalencodings as defaults.
Layers it makes sense to include in the PERLIO environmentvariable are briefly summarized below. For more details see PerlIO.
- :bytes
  A pseudolayer that turns the :utf8 flag off for the layer below;unlikely to be useful on its own in the global PERLIO environment variable.You perhaps were thinking of :crlf:bytes or :perlio:bytes.
- :crlf
  A layer which does CRLF to "\n" translation distinguishing "text" and"binary" files in the manner of MS-DOS and similar operating systems.(It currently does not mimic MS-DOS as far as treating of Control-Zas being an end-of-file marker.)
- :mmap
  A layer that implements "reading" of files by using mmap(2) tomake an entire file appear in the process's address space, and thenusing that as PerlIO's "buffer".
- :perlio
  This is a re-implementation of stdio-like buffering written as aPerlIO layer. As such it will call whatever layer is below it forits operations, typically :unix.
- :pop
  An experimental pseudolayer that removes the topmost layer.Use with the same care as is reserved for nitroglycerine.
- :raw
  A pseudolayer that manipulates other layers. Applying the :rawlayer is equivalent to calling binmode($fh). It makes the streampass each byte as-is without translation. In particular, both CRLFtranslation and intuiting :utf8 from the locale are disabled.
  Unlike in earlier versions of Perl, :raw is notjust the inverse of :crlf: other layers which would affect thebinary nature of the stream are also removed or disabled.
- :stdio
  This layer provides a PerlIO interface by wrapping system's ANSI C "stdio"library calls. The layer provides both buffering and IO.Note that the :stdio layer does not do CRLF translation even if thatis the platform's normal behaviour. You will need a :crlf layer above itto do that.
- :unix
  Low-level layer that calls read, write, lseek, etc.
- :utf8
  A pseudolayer that enables a flag in the layer below to tell Perlthat output should be in utf8 and that input should be regarded asalready in valid utf8 form. WARNING: It does not check for validity and as suchshould be handled with extreme caution for input, because security violationscan occur with non-shortest UTF-8 encodings, etc. Generally :encoding(utf8) isthe best option when reading UTF-8 encoded data.
- :win32
  On Win32 platforms this experimental layer uses native "handle" IOrather than a Unix-like numeric file descriptor layer. Known to bebuggy in this release (5.14).
The default set of layers should give acceptable results on all platforms
For Unix platforms that will be the equivalent of "unix perlio" or "stdio".Configure is set up to prefer the "stdio" implementation if the system's libraryprovides for fast access to the buffer; otherwise, it uses the "unix perlio"implementation.
On Win32 the default in this release (5.14) is "unix crlf". Win32's "stdio"has a number of bugs/mis-features for Perl IO which are somewhat dependingon the version and vendor of the C compiler. Using our own crlf layer asthe buffer avoids those issues and makes things more uniform. The crlflayer provides CRLF conversion as well as buffering.
This release (5.14) uses unix as the bottom layer on Win32, and so stilluses the C compiler's numeric file descriptor routines. There is anexperimental native win32 layer, which is expected to be enhanced andshould eventually become the default under Win32.
The PERLIO environment variable is completely ignored when Perlis run in taint mode.
PERLIO_DEBUG
If set to the name of a file or device, certain operations of PerlIOsubsystem will be logged to that file, which is opened in append modeTypical uses are in Unix:
```
   % env PERLIO_DEBUG=/dev/tty perl script ...
 
```
and under Win32, the approximately equivalent:
```
   > set PERLIO_DEBUG=CON
    perl script ...
 
```
This functionality is disabled for setuid scripts and for scripts runwith -T.
PERLLIB
A list of directories in which to look for Perl libraryfiles before looking in the standard library and the current directory.If PERL5LIB is defined, PERLLIB is not used.
The PERLLIB environment variable is completely ignored when Perlis run in taint mode.
PERL5DB
The command used to load the debugger code. The default is:
```
BEGIN { require "perl5db.pl" }
 
```
The PERL5DB environment variable is only used when Perl is started witha bare -d switch.
PERL5DB_THREADED
If set to a true value, indicates to the debugger that the code beingdebugged uses threads.
PERL5SHELL (specific to the Win32 port)
On Win32 ports only, may be set to an alternative shell that Perl must useinternally for executing "backtick" commands or system(). Default iscmd.exe /x/d/c on WindowsNT and command.com /c on Windows95. Thevalue is considered space-separated. Precede any character thatneeds to be protected, like a space or backslash, with another backslash.
Note that Perl doesn't use COMSPEC for this purpose becauseCOMSPEC has a high degree of variability among users, leading toportability concerns. Besides, Perl can use a shell that may not befit for interactive use, and setting COMSPEC to such a shell mayinterfere with the proper functioning of other programs (which usuallylook in COMSPEC to find a shell fit for interactive use).
Before Perl 5.10.0 and 5.8.8, PERL5SHELL was not taint checkedwhen running external commands. It is recommended thatyou explicitly set (or delete) $ENV{PERL5SHELL} when runningin taint mode under Windows.
PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)
Set to 1 to allow the use of non-IFS compatible LSPs (Layered Service Providers).Perl normally searches for an IFS-compatible LSP because this is requiredfor its emulation of Windows sockets as real filehandles. However, this maycause problems if you have a firewall such as McAfee Guardian, which requiresthat all applications use its LSP but which is not IFS-compatible, because clearlyPerl will normally avoid using such an LSP.
Setting this environment variable to 1 means that Perl will simply use thefirst suitable LSP enumerated in the catalog, which keeps McAfee Guardianhappy--and in that particular case Perl still works too because McAfeeGuardian's LSP actually plays other games which allow applicationsrequiring IFS compatibility to work.
PERL_DEBUG_MSTATS
Relevant only if Perl is compiled with the malloc included with the Perldistribution; that is, if perl -V:d_mymalloc is "define".
If set, this dumps out memory statistics after execution. If setto an integer greater than one, also dumps out memory statisticsafter compilation.
PERL_DESTRUCT_LEVEL
Relevant only if your Perl executable was built with -DDEBUGGING,this controls the behaviour of global destruction of objects and otherreferences. See PERL_DESTRUCT_LEVEL in perlhacktips for more information.
PERL_DL_NONLAZY
Set to "1" to have Perl resolve all undefined symbols when it loadsa dynamic library. The default behaviour is to resolve symbols whenthey are used. Setting this variable is useful during testing ofextensions, as it ensures that you get an error on misspelled functionnames even if the test suite doesn't call them.
PERL_ENCODING
If using the use encoding pragma without an explicit encoding name, thePERL_ENCODING environment variable is consulted for an encoding name.
PERL_HASH_SEED
(Since Perl 5.8.1.) Used to randomize Perl's internal hash function.To emulate the pre-5.8.1 behaviour, set to an integer; "0" meansexactly the same order as in 5.8.0. "Pre-5.8.1" means, among otherthings, that hash keys will always have the same ordering betweendifferent runs of Perl.
Most hashes by default return elements in the same order as in Perl 5.8.0.On a hash by hash basis, if pathological data is detected during a hashkey insertion, then that hash will switch to an alternative random hashseed.
The default behaviour is to randomize unless the PERL_HASH_SEED is set.If Perl has been compiled with -DUSE_HASH_SEED_EXPLICIT, the defaultbehaviour is not to randomize unless the PERL_HASH_SEED is set.
If PERL_HASH_SEED is unset or set to a non-numeric string, Perl usesthe pseudorandom seed supplied by the operating system and libraries.
PLEASE NOTE: The hash seed is sensitive information. Hashes arerandomized to protect against local and remote attacks against Perlcode. By manually setting a seed, this protection may be partially orcompletely lost.
See Algorithmic Complexity Attacks in perlsec andPERL_HASH_SEED_DEBUG for more information.
PERL_HASH_SEED_DEBUG
(Since Perl 5.8.1.) Set to "1" to display (to STDERR) the value ofthe hash seed at the beginning of execution. This, combined withPERL_HASH_SEED is intended to aid in debugging nondeterministicbehaviour caused by hash randomization.
Note that the hash seed is sensitive information: by knowing it, onecan craft a denial-of-service attack against Perl code, even remotely;see Algorithmic Complexity Attacks in perlsec for more information.Do not disclose the hash seed to people who don't need to know it.See also hash_seed() in Hash::Util.
PERL_MEM_LOG
If your Perl was configured with -Accflags=-DPERL_MEM_LOG, settingthe environment variable PERL_MEM_LOG enables logging debugmessages. The value has the form <number>[m][s][t], wherenumber is the file descriptor number you want to write to (2 isdefault), and the combination of letters specifies that you wantinformation about (m)emory and/or (s)v, optionally with(t)imestamps. For example, PERL_MEM_LOG=1mst logs allinformation to stdout. You can write to other opened file descriptorsin a variety of ways:
```
  $ 3>foo3 PERL_MEM_LOG=3m perl ...
 
```
PERL_ROOT (specific to the VMS port)
A translation-concealed rooted logical name that contains Perl and thelogical device for the @INC path on VMS only. Other logical names thataffect Perl on VMS include PERLSHR, PERL_ENV_TABLES, andSYS$TIMEZONE_DIFFERENTIAL, but are optional and discussed further inperlvms and in README.vms in the Perl source distribution.
PERL_SIGNALS
Available in Perls 5.8.1 and later. If set to "unsafe", the pre-Perl-5.8.0signal behaviour (which is immediate but unsafe) is restored. If setto safe, then safe (but deferred) signals are used. SeeDeferred Signals (Safe Signals) in perlipc.
PERL_UNICODE
Equivalent to the -C command-line switch. Note that this is nota boolean variable. Setting this to "1" is not the right way to"enable Unicode" (whatever that would mean). You can use "0" to"disable Unicode", though (or alternatively unset PERL_UNICODE inyour shell before starting Perl). See the description of the -Cswitch for more information.
SYS$LOGIN (specific to the VMS port)
Used if chdir has no argument and HOME and LOGDIR are not set.

Perl also has environment variables that control how Perl handles dataspecific to particular natural languages; see perllocale.

Perl and its various modules and components, including its test frameworks,may sometimes make use of certain other environment variables. Some ofthese are specific to a particular platform. Please consult theappropriate module documentation and any documentation for your platform(like perlsolaris, perllinux, perlmacosx, perlwin32, etc) forvariables peculiar to those specific situations.

Perl makes all environment variables available to the program beingexecuted, and passes these along to any child processes it starts.However, programs running setuid would do well to execute the followinglines before doing anything else, just to keep people honest:

 $ENV{PATH}  = "/bin:/usr/bin"; # or whatever you need
  $ENV{SHELL} = "/bin/sh" if exists $ENV{SHELL};
  delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};

Page index

NAME
SYNOPSIS
DESCRIPTION
ENVIRONMENT

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