A Perl program consists of a sequence of declarations and statementswhich run from the top to the bottom. Loops, subroutines, and othercontrol structures allow you to jump around within the code.

Perl is a free-form language: you can format and indent it howeveryou like. Whitespace serves mostly to separate tokens, unlikelanguages like Python where it is an important part of the syntax,or Fortran where it is immaterial.

Many of Perl's syntactic elements are optional. Rather thanrequiring you to put parentheses around every function call anddeclare every variable, you can often leave such explicit elements offand Perl will figure out what you meant. This is known as Do What IMean, abbreviated DWIM. It allows programmers to be lazy and tocode in a style with which they are comfortable.

Perl borrows syntax and concepts from many languages: awk, sed, C,Bourne Shell, Smalltalk, Lisp and even English. Otherlanguages have borrowed syntax from Perl, particularly its regularexpression extensions. So if you have programmed in another languageyou will see familiar pieces in Perl. They often work the same, butsee perltrap for information about how they differ.

Declarations

The only things you need to declare in Perl are report formats andsubroutines (and sometimes not even subroutines). A scalar variable holdsthe undefined value (undef) until it has been assigned a definedvalue, which is anything other than undef. When used as a number,undef is treated as 0; when used as a string, it is treated asthe empty string, ""; and when used as a reference that isn't beingassigned to, it is treated as an error. If you enable warnings,you'll be notified of an uninitialized value whenever you treatundef as a string or a number. Well, usually. Boolean contexts,such as:

 if ($a) {}

are exempt from warnings (because they care about truth rather thandefinedness). Operators such as ++, --, +=,-=, and .=, that operate on undefined variables such as:

 undef $a;
  $a++;

are also always exempt from such warnings.

A declaration can be put anywhere a statement can, but has no effect onthe execution of the primary sequence of statements: declarations alltake effect at compile time. All declarations are typically put atthe beginning or the end of the script. However, if you're usinglexically-scoped private variables created with my(),state(), or our(), you'll have to make sureyour format or subroutine definition is within the same block scopeas the my if you expect to be able to access those private variables.

Declaring a subroutine allows a subroutine name to be used as if it were alist operator from that point forward in the program. You can declare asubroutine without defining it by saying sub name, thus:

 sub myname;
  $me = myname $0 or die "can't get myname";

A bare declaration like that declares the function to be a list operator,not a unary operator, so you have to be careful to use parentheses (oror instead of ||.) The || operator binds too tightly to use afterlist operators; it becomes part of the last element. You can always useparentheses around the list operators arguments to turn the list operatorback into something that behaves more like a function call. Alternatively,you can use the prototype ($) to turn the subroutine into a unaryoperator:

  sub myname ($);
   $me = myname $0 || die "can't get myname";

That now parses as you'd expect, but you still ought to get in the habit ofusing parentheses in that situation. For more on prototypes, seeperlsub

Subroutines declarations can also be loaded up with the require statementor both loaded and imported into your namespace with a use statement.See perlmod for details on this.

A statement sequence may contain declarations of lexically-scopedvariables, but apart from declaring a variable name, the declaration actslike an ordinary statement, and is elaborated within the sequence ofstatements as if it were an ordinary statement. That means it actuallyhas both compile-time and run-time effects.

Comments

Text from a "#" character until the end of the line is a comment,and is ignored. Exceptions include "#" inside a string or regularexpression.

Simple Statements

The only kind of simple statement is an expression evaluated for itsside-effects. Every simple statement must be terminated with asemicolon, unless it is the final statement in a block, in which casethe semicolon is optional. But put the semicolon in anyway if theblock takes up more than one line, because you may eventually addanother line. Note that there are operators like eval {}, sub {}, anddo {} that look like compound statements, but aren't--they're justTERMs in an expression--and thus need an explicit termination when usedas the last item in a statement.

Truth and Falsehood

The number 0, the strings '0' and "", the empty list (), andundef are all false in a boolean context. All other values are true.Negation of a true value by ! or not returns a special false value.When evaluated as a string it is treated as "", but as a number, itis treated as 0. Most Perl operatorsthat return true or false behave this way.

Statement Modifiers

Any simple statement may optionally be followed by a SINGLE modifier,just before the terminating semicolon (or block ending). The possiblemodifiers are:

 if EXPR
  unless EXPR
  while EXPR
  until EXPR
  for LIST
  foreach LIST
  when EXPR

The EXPR following the modifier is referred to as the "condition".Its truth or falsehood determines how the modifier will behave.

if executes the statement once if and only if the condition istrue. unless is the opposite, it executes the statement unlessthe condition is true (that is, if the condition is false).

 print "Basset hounds got long ears" if length $ear >= 10;
  go_outside() and play() unless $is_raining;

The for(each) modifier is an iterator: it executes the statement oncefor each item in the LIST (with $_ aliased to each item in turn).

 print "Hello $_!\n" for qw(world Dolly nurse);

while repeats the statement while the condition is true.until does the opposite, it repeats the statement until thecondition is true (or while the condition is false):

 # Both of these count from 0 to 10.
  print $i++ while $i <= 10;
  print $j++ until $j >  10;

The while and until modifiers have the usual "while loop"semantics (conditional evaluated first), except when applied to ado-BLOCK (or to the Perl4 do-SUBROUTINE statement), inwhich case the block executes once before the conditional isevaluated.

This is so that you can write loops like:

 do {
 $line = <STDIN>;
 ...
  } until !defined($line) || $line eq ".\n"

See do. Note also that the loop control statements describedlater will NOT work in this construct, because modifiers don't takeloop labels. Sorry. You can always put another block inside of it(for next) or around it (for last) to do that sort of thing.For next, just double the braces:

 do {{
 next if $x == $y;
 # do something here
  }} until $x++ > $z;

For last, you have to be more elaborate:

 LOOP: { 
  do {
 last if $x = $y**2;
 # do something here
  } while $x++ <= $z;
  }

NOTE: The behaviour of a my, state, orour modified with a statement modifier conditionalor loop construct (for example, my $x if ...) isundefined. The value of the my variable may be undef, anypreviously assigned value, or possibly anything else. Don't rely onit. Future versions of perl might do something different from theversion of perl you try it out on. Here be dragons.

The when modifier is an experimental feature that first appeared in Perl5.14. To use it, you should include a use v5.14 declaration.(Technically, it requires only the switch feature, but that aspect of itwas not available before 5.14.) Operative only from within a foreachloop or a given block, it executes the statement only if the smartmatch$_ ~~ EXPR is true. If the statement executes, it is followed bya next from inside a foreach and break from inside a given.

Under the current implementation, the foreach loop can beanywhere within the when modifier's dynamic scope, but must bewithin the given block's lexical scope. This restricted maybe relaxed in a future release. See Switch Statements below.

Compound Statements

In Perl, a sequence of statements that defines a scope is called a block.Sometimes a block is delimited by the file containing it (in the caseof a required file, or the program as a whole), and sometimes a blockis delimited by the extent of a string (in the case of an eval).

But generally, a block is delimited by curly brackets, also known as braces.We will call this syntactic construct a BLOCK.

The following compound statements may be used to control flow:

 if (EXPR) BLOCK
  if (EXPR) BLOCK else BLOCK
  if (EXPR) BLOCK elsif (EXPR) BLOCK ...
  if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
 
  unless (EXPR) BLOCK
  unless (EXPR) BLOCK else BLOCK
  unless (EXPR) BLOCK elsif (EXPR) BLOCK ...
  unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
 
  given (EXPR) BLOCK
 
  LABEL while (EXPR) BLOCK
  LABEL while (EXPR) BLOCK continue BLOCK
 
  LABEL until (EXPR) BLOCK
  LABEL until (EXPR) BLOCK continue BLOCK
 
  LABEL for (EXPR; EXPR; EXPR) BLOCK
  LABEL for VAR (LIST) BLOCK
  LABEL for VAR (LIST) BLOCK continue BLOCK
 
  LABEL foreach (EXPR; EXPR; EXPR) BLOCK
  LABEL foreach VAR (LIST) BLOCK
  LABEL foreach VAR (LIST) BLOCK continue BLOCK
 
  LABEL BLOCK
  LABEL BLOCK continue BLOCK
 
  PHASE BLOCK

The experimental given statement is not automatically enabled; see Switch Statements below for how to do so, and the attendant caveats.

Unlike in C and Pascal, in Perl these are all defined in terms of BLOCKs,not statements. This means that the curly brackets are required--nodangling statements allowed. If you want to write conditionals withoutcurly brackets, there are several other ways to do it. The followingall do the same thing:

 if (!open(FOO)) { die "Can't open $FOO: $!" }
  die "Can't open $FOO: $!" unless open(FOO);
  open(FOO)  || die "Can't open $FOO: $!";
  open(FOO) ? () : die "Can't open $FOO: $!";
 # a bit exotic, that last one

The if statement is straightforward. Because BLOCKs are alwaysbounded by curly brackets, there is never any ambiguity about whichif an else goes with. If you use unless in place of if,the sense of the test is reversed. Like if, unless can be followedby else. unless can even be followed by one or more elsifstatements, though you may want to think twice before using that particularlanguage construct, as everyone reading your code will have to think at leasttwice before they can understand what's going on.

The while statement executes the block as long as the expression istrue.The until statement executes the block as long as the expression isfalse.The LABEL is optional, and if present, consists of an identifier followedby a colon. The LABEL identifies the loop for the loop controlstatements next, last, and redo.If the LABEL is omitted, the loop control statementrefers to the innermost enclosing loop. This may include dynamicallylooking back your call-stack at run time to find the LABEL. Suchdesperate behavior triggers a warning if you use the use warningspragma or the -w flag.

If there is a continue BLOCK, it is always executed just before theconditional is about to be evaluated again. Thus it can be used toincrement a loop variable, even when the loop has been continued viathe next statement.

When a block is preceding by a compilation phase keyword such as BEGIN,END, INIT, CHECK, or UNITCHECK, then the block will run onlyduring the corresponding phase of execution. See perlmod for more details.

Extension modules can also hook into the Perl parser to define newkinds of compound statements. These are introduced by a keyword whichthe extension recognizes, and the syntax following the keyword isdefined entirely by the extension. If you are an implementor, seePL_keyword_plugin in perlapi for the mechanism. If you are using sucha module, see the module's documentation for details of the syntax thatit defines.

Loop Control

The next command starts the next iteration of the loop:

 LINE: while (<STDIN>) {
 next LINE if /^#/;# discard comments
 ...
  }

The last command immediately exits the loop in question. Thecontinue block, if any, is not executed:

 LINE: while (<STDIN>) {
 last LINE if /^$/;# exit when done with header
 ...
  }

The redo command restarts the loop block without evaluating theconditional again. The continue block, if any, is not executed.This command is normally used by programs that want to lie to themselvesabout what was just input.

For example, when processing a file like /etc/termcap.If your input lines might end in backslashes to indicate continuation, youwant to skip ahead and get the next record.

 while (<>) {
 chomp;
 if (s/\$//) {
  $_ .= <>;
  redo unless eof();
 }
 # now process $_
  }

which is Perl shorthand for the more explicitly written version:

 LINE: while (defined($line = <ARGV>)) {
 chomp($line);
 if ($line =~ s/\$//) {
  $line .= <ARGV>;
  redo LINE unless eof(); # not eof(ARGV)!
 }
 # now process $line
  }

Note that if there were a continue block on the above code, it wouldget executed only on lines discarded by the regex (since redo skips thecontinue block). A continue block is often used to reset line countersor m?pat? one-time matches:

 # inspired by :1,$g/fred/s//WILMA/
  while (<>) {
 m?(fred)? && s//WILMA $1 WILMA/;
 m?(barney)?  && s//BETTY $1 BETTY/;
 m?(homer)?   && s//MARGE $1 MARGE/;
  } continue {
 print "$ARGV $.: $_";
 close ARGV  if eof;# reset $.
 reset if eof;# reset ?pat?
  }

If the word while is replaced by the word until, the sense of thetest is reversed, but the conditional is still tested before the firstiteration.

Loop control statements don't work in an if or unless, sincethey aren't loops. You can double the braces to make them such, though.

 if (/pattern/) {{
 last if /fred/;
 next if /barney/; # same effect as "last",
   # but doesn't document as well
 # do something here
  }}

This is caused by the fact that a block by itself acts as a loop thatexecutes once, see Basic BLOCKs.

The form while/if BLOCK BLOCK, available in Perl 4, is no longeravailable. Replace any occurrence of if BLOCK by if (do BLOCK).

For Loops

Perl's C-style for loop works like the corresponding while loop;that means that this:

 for ($i = 1; $i < 10; $i++) {
 ...
  }

is the same as this:

 $i = 1;
  while ($i < 10) {
 ...
  } continue {
 $i++;
  }

There is one minor difference: if variables are declared with myin the initialization section of the for, the lexical scope ofthose variables is exactly the for loop (the body of the loopand the control sections).

Besides the normal array index looping, for can lend itselfto many other interesting applications. Here's one that avoids theproblem you get into if you explicitly test for end-of-file onan interactive file descriptor causing your program to appear tohang.

 $on_a_tty = -t STDIN && -t STDOUT;
  sub prompt { print "yes? " if $on_a_tty }
  for ( prompt(); <STDIN>; prompt() ) {
 # do something
  }

Using readline (or the operator form, <EXPR>) as theconditional of a for loop is shorthand for the following. Thisbehaviour is the same as a while loop conditional.

 for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
  # do something
  }

Foreach Loops

The foreach loop iterates over a normal list value and sets thevariable VAR to be each element of the list in turn. If the variableis preceded with the keyword my, then it is lexically scoped, andis therefore visible only within the loop. Otherwise, the variable isimplicitly local to the loop and regains its former value upon exitingthe loop. If the variable was previously declared with my, it usesthat variable instead of the global one, but it's still localized tothe loop. This implicit localization occurs only in a foreachloop.

The foreach keyword is actually a synonym for the for keyword, soyou can use either. If VAR is omitted, $_ is set to each value.

If any element of LIST is an lvalue, you can modify it by modifyingVAR inside the loop. Conversely, if any element of LIST is NOT anlvalue, any attempt to modify that element will fail. In other words,the foreach loop index variable is an implicit alias for each itemin the list that you're looping over.

If any part of LIST is an array, foreach will get very confused ifyou add or remove elements within the loop body, for example withsplice. So don't do that.

foreach probably won't do what you expect if VAR is a tied or otherspecial variable. Don't do that either.

Examples:

 for (@ary) { s/foo/bar/ }
 
  for my $elem (@elements) {
 $elem *= 2;
  }
 
  for $count (reverse(1..10), "BOOM") {
 print $count, "\n";
 sleep(1);
  }
 
  for (1..15) { print "Merry Christmas\n"; }
 
  foreach $item (split(/:[\n:]*/, $ENV{TERMCAP})) {
 print "Item: $item\n";
  }

Here's how a C programmer might code up a particular algorithm in Perl:

 for (my $i = 0; $i < @ary1; $i++) {
 for (my $j = 0; $j < @ary2; $j++) {
  if ($ary1[$i] > $ary2[$j]) {
 last; # can't go to outer :-(
  }
  $ary1[$i] += $ary2[$j];
 }
 # this is where that last takes me
  }

Whereas here's how a Perl programmer more comfortable with the idiom mightdo it:

 OUTER: for my $wid (@ary1) {
  INNER:   for my $jet (@ary2) {
 next OUTER if $wid > $jet;
 $wid += $jet;
  }
   }

See how much easier this is? It's cleaner, safer, and faster. It'scleaner because it's less noisy. It's safer because if code gets addedbetween the inner and outer loops later on, the new code won't beaccidentally executed. The next explicitly iterates the other looprather than merely terminating the inner one. And it's faster becausePerl executes a foreach statement more rapidly than it would theequivalent for loop.

Basic BLOCKs

A BLOCK by itself (labeled or not) is semantically equivalent to aloop that executes once. Thus you can use any of the loop controlstatements in it to leave or restart the block. (Note that this isNOT true in eval{}, sub{}, or contrary to popular beliefdo{} blocks, which do NOT count as loops.) The continueblock is optional.

The BLOCK construct can be used to emulate case structures.

 SWITCH: {
 if (/^abc/) { $abc = 1; last SWITCH; }
 if (/^def/) { $def = 1; last SWITCH; }
 if (/^xyz/) { $xyz = 1; last SWITCH; }
 $nothing = 1;
  }

You'll also find that foreach loop used to create a topicalizerand a switch:

 SWITCH:
  for ($var) {
 if (/^abc/) { $abc = 1; last SWITCH; }
 if (/^def/) { $def = 1; last SWITCH; }
 if (/^xyz/) { $xyz = 1; last SWITCH; }
 $nothing = 1;
  }

Such constructs are quite frequently used, both because older versions ofPerl had no official switch statement, and also because the new versiondescribed immediately below remains experimental and can sometimes be confusing.

Switch Statements

Starting from Perl 5.10.1 (well, 5.10.0, but it didn't workright), you can say

 use feature "switch";

to enable an experimental switch feature. This is loosely based on anold version of a Perl 6 proposal, but it no longer resembles the Perl 6construct. You also get the switch feature whenever you declare that yourcode prefers to run under a version of Perl that is 5.10 or later. Forexample:

 use v5.14;

Under the "switch" feature, Perl gains the experimental keywordsgiven, when, default, continue, and break.Starting from Perl 5.16, one can prefix the switchkeywords with CORE:: to access the feature without a use featurestatement. The keywords given andwhen are analogous to switch andcase in other languages, so the code in the previous section could berewritten as

 use v5.10.1;
  for ($var) {
 when (/^abc/) { $abc = 1 }
 when (/^def/) { $def = 1 }
 when (/^xyz/) { $xyz = 1 }
 default   { $nothing = 1 }
  }

The foreach is the non-experimental way to set a topicalizer.If you wish to use the highly experimental given, that could bewritten like this:

 use v5.10.1;
  given ($var) {
 when (/^abc/) { $abc = 1 }
 when (/^def/) { $def = 1 }
 when (/^xyz/) { $xyz = 1 }
 default   { $nothing = 1 }
  }

As of 5.14, that can also be written this way:

 use v5.14;
  for ($var) {
 $abc = 1 when /^abc/;
 $def = 1 when /^def/;
 $xyz = 1 when /^xyz/;
 default { $nothing = 1 }
  }

Or if you don't care to play it safe, like this:

 use v5.14;
  given ($var) {
 $abc = 1 when /^abc/;
 $def = 1 when /^def/;
 $xyz = 1 when /^xyz/;
 default { $nothing = 1 }
  }

The arguments to given and when are in scalar context,and given assigns the $_ variable its topic value.

Exactly what the EXPR argument to when does is hard to describeprecisely, but in general, it tries to guess what you want done. Sometimesit is interpreted as $_ ~~ EXPR, and sometimes it does not. Italso behaves differently when lexically enclosed by a given block thanit does when dynamically enclosed by a foreach loop. The rules are fartoo difficult to understand to be described here. See Experimental Details on given and when later on.

Due to an unfortunate bug in how given was implemented between Perl 5.10and 5.14, under those implementations the version of $_ governed bygiven is merely a lexically scoped copy of the original, not adynamically scoped alias to the original, as it would be if it were aforeach or under both the original and the current Perl 6 languagespecification. This bug is expected to be addressed in a future release ofPerl. For forwards compatibility, if you really want a lexical $_,specify that explicitly:

 given(my $_ = EXPR) { ... }

In the meanwhile, stick to foreach for your topicalizer andyou will be less unhappy.

Goto

Although not for the faint of heart, Perl does support a gotostatement. There are three forms: goto-LABEL, goto-EXPR, andgoto-&NAME. A loop's LABEL is not actually a valid target fora goto; it's just the name of the loop.

The goto-LABEL form finds the statement labeled with LABEL and resumesexecution there. It may not be used to go into any construct thatrequires initialization, such as a subroutine or a foreach loop. Italso can't be used to go into a construct that is optimized away. Itcan be used to go almost anywhere else within the dynamic scope,including out of subroutines, but it's usually better to use some otherconstruct such as last or die. The author of Perl has never felt theneed to use this form of goto (in Perl, that is--C is another matter).

The goto-EXPR form expects a label name, whose scope will be resolveddynamically. This allows for computed gotos per FORTRAN, but isn'tnecessarily recommended if you're optimizing for maintainability:

 goto(("FOO", "BAR", "GLARCH")[$i]);

The goto-&NAME form is highly magical, and substitutes a call to thenamed subroutine for the currently running subroutine. This is used byAUTOLOAD() subroutines that wish to load another subroutine and thenpretend that the other subroutine had been called in the first place(except that any modifications to @_ in the current subroutine arepropagated to the other subroutine.) After the goto, not even caller()will be able to tell that this routine was called first.

In almost all cases like this, it's usually a far, far better idea to use thestructured control flow mechanisms of next, last, or redo instead ofresorting to a goto. For certain applications, the catch and throw pair ofeval{} and die() for exception processing can also be a prudent approach.

The Ellipsis Statement

Beginning in Perl 5.12, Perl accepts an ellipsis, "...", as aplaceholder for code that you haven't implemented yet. This form ofellipsis, the unimplemented statement, should not be confused with thebinary flip-flop ... operator. One is a statement and the other anoperator. (Perl doesn't usually confuse them because usually Perl can tellwhether it wants an operator or a statement, but see below for exceptions.)

When Perl 5.12 or later encounters an ellipsis statement, it parses thiswithout error, but if and when you should actually try to execute it, Perlthrows an exception with the text Unimplemented:

 use v5.12;
  sub unimplemented { ... }
  eval { unimplemented() };
  if ($@ =~ /^Unimplemented at /) {
 say "I found an ellipsis!";
  }

You can only use the elliptical statement to stand in for acomplete statement. These examples of how the ellipsis works:

 use v5.12;
  { ... }
  sub foo { ... }
  ...;
  eval { ... };
  sub somemeth {
 my $self = shift;
 ...;
  }
  $x = do {
 my $n;
 ...;
 say "Hurrah!";
 $n;
  };

The elliptical statement cannot stand in for an expression thatis part of a larger statement, since the ... is also the three-dotversion of the flip-flop operator (see Range Operators in perlop).

These examples of attempts to use an ellipsis are syntax errors:

 use v5.12;
 
  print ...;
  open(my $fh, ">", "/dev/passwd") or ...;
  if ($condition && ... ) { say "Howdy" };

There are some cases where Perl can't immediately tell the differencebetween an expression and a statement. For instance, the syntax for ablock and an anonymous hash reference constructor look the same unlessthere's something in the braces to give Perl a hint. The ellipsis is asyntax error if Perl doesn't guess that the { ... } is a block. In thatcase, it doesn't think the ... is an ellipsis because it's expecting anexpression instead of a statement:

 @transformed = map { ... } @input;  # syntax error

You can use a ; inside your block to denote that the { ... } is ablock and not a hash reference constructor. Now the ellipsis works:

 @transformed = map {; ... } @input; # ; disambiguates
 
  @transformed = map { ...; } @input; # ; disambiguates

Note: Some folks colloquially refer to this bit of punctuation as a"yada-yada" or "triple-dot", but its true nameis actually an ellipsis. Perl does not yetaccept the Unicode version, U+2026 HORIZONTAL ELLIPSIS, as an alias for..., but someday it may.

PODs: Embedded Documentation

Perl has a mechanism for intermixing documentation with source code.While it's expecting the beginning of a new statement, if the compilerencounters a line that begins with an equal sign and a word, like this

 =head1 Here There Be Pods!

Then that text and all remaining text up through and including a linebeginning with =cut will be ignored. The format of the interveningtext is described in perlpod.

This allows you to intermix your source codeand your documentation text freely, as in

 =item snazzle($)
 
  The snazzle() function will behave in the most spectacular
  form that you can possibly imagine, not even excepting
  cybernetic pyrotechnics.
 
  =cut back to the compiler, nuff of this pod stuff!
 
  sub snazzle($) {
 my $thingie = shift;
 .........
  }

Note that pod translators should look at only paragraphs beginningwith a pod directive (it makes parsing easier), whereas the compileractually knows to look for pod escapes even in the middle of aparagraph. This means that the following secret stuff will beignored by both the compiler and the translators.

 $a=3;
  =secret stuff
  warn "Neither POD nor CODE!?"
  =cut back
  print "got $a\n";

You probably shouldn't rely upon the warn() being podded out forever.Not all pod translators are well-behaved in this regard, and perhapsthe compiler will become pickier.

One may also use pod directives to quickly comment out a sectionof code.

Plain Old Comments (Not!)

Perl can process line directives, much like the C preprocessor. Usingthis, one can control Perl's idea of filenames and line numbers inerror or warning messages (especially for strings that are processedwith eval()). The syntax for this mechanism is almost the same as formost C preprocessors: it matches the regular expression

 # example: '# line 42 "new_filename.plx"'
  /^\#   \s*
     line \s+ (\d+)   \s*
     (?:\s("?)([^"]+)\g2)? \s*
   $/x

with $1 being the line number for the next line, and $3 beingthe optional filename (specified with or without quotes). Note thatno whitespace may precede the #, unlike modern C preprocessors.

There is a fairly obvious gotcha included with the line directive:Debuggers and profilers will only show the last source line to appearat a particular line number in a given file. Care should be taken notto cause line number collisions in code you'd like to debug later.

Here are some examples that you should be able to type into your commandshell:

 % perl
  # line 200 "bzzzt"
  # the '#' on the previous line must be the first char on line
  die 'foo';
  __END__
  foo at bzzzt line 201.
 
  % perl
  # line 200 "bzzzt"
  eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
  __END__
  foo at - line 2001.
 
  % perl
  eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
  __END__
  foo at foo bar line 200.
 
  % perl
  # line 345 "goop"
  eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"
  print $@;
  __END__
  foo at goop line 345.

Experimental Details on given and when

As previously mentioned, the "switch" feature is considered highlyexperimental; it is subject to change with little notice. In particular,both given and when have tricky behaviours that are expected tochange to become less tricky in the future. Do not rely upon theircurrent (mis)implementations.

Here is a longer example of given:

 use feature ":5.10";
  given ($foo) {
 when (undef) {
  say '$foo is undefined';
 }
 when ("foo") {
  say '$foo is the string "foo"';
 }
 when ([1,3,5,7,9]) {
  say '$foo is an odd digit';
  continue; # Fall through
 }
 when ($_ < 100) {
  say '$foo is numerically less than 100';
 }
 when (\&complicated_check) {
  say 'a complicated check for $foo is true';
 }
 default {
  die q(I don't know what to do with $foo);
 }
  }

As currently implemented, given(EXPR) assign the value of EXPR tomerely a lexically scoped copy (!) of $_, not a dynamicallyscoped alias the way foreach does. That makes it similar to

do { my $_ = EXPR; ... }

except that the block is automatically broken out of by asuccessful when or an explicit break. Because it is only acopy, and because it is only lexically scoped, not dynamicallyscoped, you cannot do the things with it that you are used to ina foreach loop. In particular, you probably cannot usearbitrary function calls. Best stick to foreach for that.

Most of the power comes from the implicit smartmatching that cansometimes apply. Most of the time, when(EXPR) is treated as animplicit smartmatch of $_, that is, $_ ~~ EXPR. (SeeSmartmatch Operator in perlop for more information on smartmatching.)But when EXPR is one of the 10 exceptional cases (or things like them)listed below, it is used directly as a boolean.

1.

A user-defined subroutine call or a method invocation.

2.

A regular expression match in the form of /REGEX/, $foo =~ /REGEX/,or $foo =~ EXPR. Also, a negated regular expression match inthe form !/REGEX/, $foo !~ /REGEX/, or $foo !~ EXPR.

3.

A smart match that uses an explicit ~~ operator, such as EXPR ~~ EXPR.

4.

A boolean comparison operator such as $_ < 10 or $x eq "abc" Therelational operators that this applies to are the six numeric comparisons(<, >, <=, >=, ==, and !=), andthe six string comparisons (lt, gt, le, ge, eq, and ne).

NOTE: You will often have to use $c ~~ $_ becausethe default case uses $_ ~~ $c , which is frequentlythe opposite of what you want.

5.

At least the three builtin functions defined(...), exists(...), andeof(...). We might someday add more of these later if we think of them.

6.

A negated expression, whether !(EXPR) or not(EXPR), or a logicalexclusive-or, (EXPR1) xor (EXPR2). The bitwise versions (~ and ^)are not included.

7.

A filetest operator, with exactly 4 exceptions: -s, -M, -A, and-C, as these return numerical values, not boolean ones. The -zfiletest operator is not included in the exception list.

8.

The .. and ... flip-flop operators. Note that the ... flip-flopoperator is completely different from the ... elliptical statementjust described.

In those 8 cases above, the value of EXPR is used directly as a boolean, sono smartmatching is done. You may think of when as a smartsmartmatch.

Furthermore, Perl inspects the operands of logical operators todecide whether to use smartmatching for each one by applying theabove test to the operands:

9.: If EXPR is EXPR1 && EXPR2 or EXPR1 and EXPR2, the test is appliedrecursively to both EXPR1 and EXPR2.Only if both operands also pass thetest, recursively, will the expression be treated as boolean. Otherwise,smartmatching is used.
10.: If EXPR is EXPR1 || EXPR2, EXPR1 // EXPR2, or EXPR1 or EXPR2, thetest is applied recursively to EXPR1 only (which might itself be ahigher-precedence AND operator, for example, and thus subject to theprevious rule), not to EXPR2. If EXPR1 is to use smartmatching, then EXPR2also does so, no matter what EXPR2 contains. But if EXPR2 does not get touse smartmatching, then the second argument will not be either. This isquite different from the && case just described, so be careful.

These rules are complicated, but the goal is for them to do what you want(even if you don't quite understand why they are doing it). For example:

 when (/^\d+$/ && $_ < 75) { ... }

will be treated as a boolean match because the rules say botha regex match and an explicit test on $_ will be treatedas boolean.

Also:

 when ([qw(foo bar)] && /baz/) { ... }

will use smartmatching because only one of the operands is a boolean:the other uses smartmatching, and that wins.

Further:

 when ([qw(foo bar)] || /^baz/) { ... }

will use smart matching (only the first operand is considered), whereas

 when (/^baz/ || [qw(foo bar)]) { ... }

will test only the regex, which causes both operands to betreated as boolean. Watch out for this one, then, because anarrayref is always a true value, which makes it effectivelyredundant. Not a good idea.

Tautologous boolean operators are still going to be optimizedaway. Don't be tempted to write

 when ("foo" or "bar") { ... }

This will optimize down to "foo", so "bar" will never be considered (eventhough the rules say to use a smartmatchon "foo"). For an alternation likethis, an array ref will work, because this will instigate smartmatching:

 when ([qw(foo bar)] { ... }

This is somewhat equivalent to the C-style switch statement's fallthroughfunctionality (not to be confused with Perl's fallthroughfunctionality--see below), wherein the same block is used for severalcase statements.

Another useful shortcut is that, if you use a literal array or hash as theargument to given, it is turned into a reference. So given(@foo) isthe same as given(\@foo), for example.

default behaves exactly like when(1 == 1), which isto say that it always matches.

Breaking out

You can use the break keyword to break out of the enclosinggiven block. Every when block is implicitly ended witha break.

Fall-through

You can use the continue keyword to fall through from onecase to the next:

 given($foo) {
 when (/x/) { say '$foo contains an x'; continue }
 when (/y/) { say '$foo contains a y' }
 default { say '$foo does not contain a y' }
  }

Return value

When a given statement is also a valid expression (for example,when it's the last statement of a block), it evaluates to:

An empty list as soon as an explicit break is encountered.
The value of the last evaluated expression of the successfulwhen/default clause, if there happens to be one.
The value of the last evaluated expression of the given block if nocondition is true.

In both last cases, the last expression is evaluated in the context thatwas applied to the given block.

Note that, unlike if and unless, failed when statements alwaysevaluate to an empty list.

 my $price = do {
 given ($item) {
  when (["pear", "apple"]) { 1 }
  break when "vote";  # My vote cannot be bought
  1e10  when /Mona Lisa/;
  "unknown";
 }
  };

Currently, given blocks can't alwaysbe used as proper expressions. Thismay be addressed in a future version of Perl.

Switching in a loop

Instead of using given(), you can use a foreach() loop.For example, here's one way to count how many times a particularstring occurs in an array:

 use v5.10.1;
  my $count = 0;
  for (@array) {
 when ("foo") { ++$count }
  }
  print "\@array contains $count copies of 'foo'\n";

Or in a more recent version:

 use v5.14;
  my $count = 0;
  for (@array) {
 ++$count when "foo";
  }
  print "\@array contains $count copies of 'foo'\n";

At the end of all when blocks, there is an implicit next.You can override that with an explicit last if you'reinterested in only the first match alone.

This doesn't work if you explicitly specify a loop variable, asin for $item (@array). You have to use the default variable $_.

Differences from Perl 6

The Perl 5 smartmatch and given/when constructs are not compatiblewith their Perl 6 analogues. The most visible difference and leastimportant difference is that, in Perl 5, parentheses are required aroundthe argument to given() and when() (except when this last one is usedas a statement modifier). Parentheses in Perl 6 are always optional in acontrol construct such as if(), while(), or when(); they can't bemade optional in Perl 5 without a great deal of potential confusion,because Perl 5 would parse the expression

 given $foo {
 ...
  }

as though the argument to given were an element of the hash%foo, interpreting the braces as hash-element syntax.

However, their are many, many other differences. For example,this works in Perl 5:

 use v5.12;
  my @primary = ("red", "blue", "green");
 
  if (@primary ~~ "red") {
  say "primary smartmatches red";
  }
 
  if ("red" ~~ @primary) {
  say "red smartmatches primary";
  }
 
  say "that's all, folks!";

But it doesn't work at all in Perl 6. Instead, you shoulduse the (parallelizable) any operator instead:

   if any(@primary) eq "red" {
    say "primary smartmatches red";
    }
 
    if "red" eq any(@primary) {
    say "red smartmatches primary";
    }

The table of smartmatches in Smartmatch Operator in perlop is notidentical to that proposed by the Perl 6 specification, mainly due todifferences between Perl 6's and Perl 5's data models, but also becausethe Perl 6 spec has changed since Perl 5 rushed into early adoption.

In Perl 6, when() will always do an implicit smartmatch with itsargument, while in Perl 5 it is convenient (albeit potentially confusing) tosuppress this implicit smartmatch in various rather loosely-definedsituations, as roughly outlined above. (The difference is largely becausePerl 5 does not have, even internally, a boolean type.)

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