8 A Perl program consists of a sequence of declarations and statements
9 which run from the top to the bottom. Loops, subroutines, and other
10 control structures allow you to jump around within the code.
12 Perl is a B<free-form> language: you can format and indent it however
13 you like. Whitespace serves mostly to separate tokens, unlike
14 languages like Python where it is an important part of the syntax,
15 or Fortran where it is immaterial.
17 Many of Perl's syntactic elements are B<optional>. Rather than
18 requiring you to put parentheses around every function call and
19 declare every variable, you can often leave such explicit elements off
20 and Perl will figure out what you meant. This is known as B<Do What I
21 Mean>, abbreviated B<DWIM>. It allows programmers to be B<lazy> and to
22 code in a style with which they are comfortable.
24 Perl B<borrows syntax> and concepts from many languages: awk, sed, C,
25 Bourne Shell, Smalltalk, Lisp and even English. Other
26 languages have borrowed syntax from Perl, particularly its regular
27 expression extensions. So if you have programmed in another language
28 you will see familiar pieces in Perl. They often work the same, but
29 see L<perltrap> for information about how they differ.
32 X<declaration> X<undef> X<undefined> X<uninitialized>
34 The only things you need to declare in Perl are report formats and
35 subroutines (and sometimes not even subroutines). A scalar variable holds
36 the undefined value (C<undef>) until it has been assigned a defined
37 value, which is anything other than C<undef>. When used as a number,
38 C<undef> is treated as C<0>; when used as a string, it is treated as
39 the empty string, C<"">; and when used as a reference that isn't being
40 assigned to, it is treated as an error. If you enable warnings,
41 you'll be notified of an uninitialized value whenever you treat
42 C<undef> as a string or a number. Well, usually. Boolean contexts,
47 are exempt from warnings (because they care about truth rather than
48 definedness). Operators such as C<++>, C<-->, C<+=>,
49 C<-=>, and C<.=>, that operate on undefined variables such as:
54 are also always exempt from such warnings.
56 A declaration can be put anywhere a statement can, but has no effect on
57 the execution of the primary sequence of statements: declarations all
58 take effect at compile time. All declarations are typically put at
59 the beginning or the end of the script. However, if you're using
60 lexically-scoped private variables created with C<my()>,
61 C<state()>, or C<our()>, you'll have to make sure
62 your format or subroutine definition is within the same block scope
63 as the my if you expect to be able to access those private variables.
65 Declaring a subroutine allows a subroutine name to be used as if it were a
66 list operator from that point forward in the program. You can declare a
67 subroutine without defining it by saying C<sub name>, thus:
68 X<subroutine, declaration>
71 $me = myname $0 or die "can't get myname";
73 A bare declaration like that declares the function to be a list operator,
74 not a unary operator, so you have to be careful to use parentheses (or
75 C<or> instead of C<||>.) The C<||> operator binds too tightly to use after
76 list operators; it becomes part of the last element. You can always use
77 parentheses around the list operators arguments to turn the list operator
78 back into something that behaves more like a function call. Alternatively,
79 you can use the prototype C<($)> to turn the subroutine into a unary
83 $me = myname $0 || die "can't get myname";
85 That now parses as you'd expect, but you still ought to get in the habit of
86 using parentheses in that situation. For more on prototypes, see
89 Subroutines declarations can also be loaded up with the C<require> statement
90 or both loaded and imported into your namespace with a C<use> statement.
91 See L<perlmod> for details on this.
93 A statement sequence may contain declarations of lexically-scoped
94 variables, but apart from declaring a variable name, the declaration acts
95 like an ordinary statement, and is elaborated within the sequence of
96 statements as if it were an ordinary statement. That means it actually
97 has both compile-time and run-time effects.
102 Text from a C<"#"> character until the end of the line is a comment,
103 and is ignored. Exceptions include C<"#"> inside a string or regular
106 =head2 Simple Statements
107 X<statement> X<semicolon> X<expression> X<;>
109 The only kind of simple statement is an expression evaluated for its
110 side-effects. Every simple statement must be terminated with a
111 semicolon, unless it is the final statement in a block, in which case
112 the semicolon is optional. But put the semicolon in anyway if the
113 block takes up more than one line, because you may eventually add
114 another line. Note that there are operators like C<eval {}>, C<sub {}>, and
115 C<do {}> that I<look> like compound statements, but aren't--they're just
116 TERMs in an expression--and thus need an explicit termination when used
117 as the last item in a statement.
119 =head2 Statement Modifiers
120 X<statement modifier> X<modifier> X<if> X<unless> X<while>
121 X<until> X<when> X<foreach> X<for>
123 Any simple statement may optionally be followed by a I<SINGLE> modifier,
124 just before the terminating semicolon (or block ending). The possible
135 The C<EXPR> following the modifier is referred to as the "condition".
136 Its truth or falsehood determines how the modifier will behave.
138 C<if> executes the statement once I<if> and only if the condition is
139 true. C<unless> is the opposite, it executes the statement I<unless>
140 the condition is true (that is, if the condition is false).
142 print "Basset hounds got long ears" if length $ear >= 10;
143 go_outside() and play() unless $is_raining;
145 The C<for(each)> modifier is an iterator: it executes the statement once
146 for each item in the LIST (with C<$_> aliased to each item in turn).
148 print "Hello $_!\n" for qw(world Dolly nurse);
150 C<while> repeats the statement I<while> the condition is true.
151 C<until> does the opposite, it repeats the statement I<until> the
152 condition is true (or while the condition is false):
154 # Both of these count from 0 to 10.
155 print $i++ while $i <= 10;
156 print $j++ until $j > 10;
158 The C<while> and C<until> modifiers have the usual "C<while> loop"
159 semantics (conditional evaluated first), except when applied to a
160 C<do>-BLOCK (or to the Perl4 C<do>-SUBROUTINE statement), in
161 which case the block executes once before the conditional is
164 This is so that you can write loops like:
169 } until !defined($line) || $line eq ".\n"
171 See L<perlfunc/do>. Note also that the loop control statements described
172 later will I<NOT> work in this construct, because modifiers don't take
173 loop labels. Sorry. You can always put another block inside of it
174 (for C<next>/C<redo>) or around it (for C<last>) to do that sort of thing.
175 X<next> X<last> X<redo>
177 For C<next> or C<redo>, just double the braces:
184 For C<last>, you have to be more elaborate and put braces around it:
194 If you need both C<next> and C<last>, you have to do both and also use a
200 last LOOP if $x == $y**2;
205 B<NOTE:> The behaviour of a C<my>, C<state>, or
206 C<our> modified with a statement modifier conditional
207 or loop construct (for example, C<my $x if ...>) is
208 B<undefined>. The value of the C<my> variable may be C<undef>, any
209 previously assigned value, or possibly anything else. Don't rely on
210 it. Future versions of perl might do something different from the
211 version of perl you try it out on. Here be dragons.
214 The C<when> modifier is an experimental feature that first appeared in Perl
215 5.14. To use it, you should include a C<use v5.14> declaration.
216 (Technically, it requires only the C<switch> feature, but that aspect of it
217 was not available before 5.14.) Operative only from within a C<foreach>
218 loop or a C<given> block, it executes the statement only if the smartmatch
219 C<< $_ ~~ I<EXPR> >> is true. If the statement executes, it is followed by
220 a C<next> from inside a C<foreach> and C<break> from inside a C<given>.
222 Under the current implementation, the C<foreach> loop can be
223 anywhere within the C<when> modifier's dynamic scope, but must be
224 within the C<given> block's lexical scope. This restriction may
225 be relaxed in a future release. See L</"Switch Statements"> below.
227 =head2 Compound Statements
228 X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace>
229 X<{> X<}> X<if> X<unless> X<given> X<while> X<until> X<foreach> X<for> X<continue>
231 In Perl, a sequence of statements that defines a scope is called a block.
232 Sometimes a block is delimited by the file containing it (in the case
233 of a required file, or the program as a whole), and sometimes a block
234 is delimited by the extent of a string (in the case of an eval).
236 But generally, a block is delimited by curly brackets, also known as braces.
237 We will call this syntactic construct a BLOCK.
239 The following compound statements may be used to control flow:
242 if (EXPR) BLOCK else BLOCK
243 if (EXPR) BLOCK elsif (EXPR) BLOCK ...
244 if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
247 unless (EXPR) BLOCK else BLOCK
248 unless (EXPR) BLOCK elsif (EXPR) BLOCK ...
249 unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
253 LABEL while (EXPR) BLOCK
254 LABEL while (EXPR) BLOCK continue BLOCK
256 LABEL until (EXPR) BLOCK
257 LABEL until (EXPR) BLOCK continue BLOCK
259 LABEL for (EXPR; EXPR; EXPR) BLOCK
260 LABEL for VAR (LIST) BLOCK
261 LABEL for VAR (LIST) BLOCK continue BLOCK
263 LABEL foreach (EXPR; EXPR; EXPR) BLOCK
264 LABEL foreach VAR (LIST) BLOCK
265 LABEL foreach VAR (LIST) BLOCK continue BLOCK
268 LABEL BLOCK continue BLOCK
272 The experimental C<given> statement is I<not automatically enabled>; see
273 L</"Switch Statements"> below for how to do so, and the attendant caveats.
275 Unlike in C and Pascal, in Perl these are all defined in terms of BLOCKs,
276 not statements. This means that the curly brackets are I<required>--no
277 dangling statements allowed. If you want to write conditionals without
278 curly brackets, there are several other ways to do it. The following
279 all do the same thing:
281 if (!open(FOO)) { die "Can't open $FOO: $!" }
282 die "Can't open $FOO: $!" unless open(FOO);
283 open(FOO) || die "Can't open $FOO: $!";
284 open(FOO) ? () : die "Can't open $FOO: $!";
285 # a bit exotic, that last one
287 The C<if> statement is straightforward. Because BLOCKs are always
288 bounded by curly brackets, there is never any ambiguity about which
289 C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
290 the sense of the test is reversed. Like C<if>, C<unless> can be followed
291 by C<else>. C<unless> can even be followed by one or more C<elsif>
292 statements, though you may want to think twice before using that particular
293 language construct, as everyone reading your code will have to think at least
294 twice before they can understand what's going on.
296 The C<while> statement executes the block as long as the expression is
298 The C<until> statement executes the block as long as the expression is
300 The LABEL is optional, and if present, consists of an identifier followed
301 by a colon. The LABEL identifies the loop for the loop control
302 statements C<next>, C<last>, and C<redo>.
303 If the LABEL is omitted, the loop control statement
304 refers to the innermost enclosing loop. This may include dynamically
305 looking back your call-stack at run time to find the LABEL. Such
306 desperate behavior triggers a warning if you use the C<use warnings>
307 pragma or the B<-w> flag.
309 If there is a C<continue> BLOCK, it is always executed just before the
310 conditional is about to be evaluated again. Thus it can be used to
311 increment a loop variable, even when the loop has been continued via
312 the C<next> statement.
314 When a block is preceding by a compilation phase keyword such as C<BEGIN>,
315 C<END>, C<INIT>, C<CHECK>, or C<UNITCHECK>, then the block will run only
316 during the corresponding phase of execution. See L<perlmod> for more details.
318 Extension modules can also hook into the Perl parser to define new
319 kinds of compound statements. These are introduced by a keyword which
320 the extension recognizes, and the syntax following the keyword is
321 defined entirely by the extension. If you are an implementor, see
322 L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
323 a module, see the module's documentation for details of the syntax that
327 X<loop control> X<loop, control> X<next> X<last> X<redo> X<continue>
329 The C<next> command starts the next iteration of the loop:
331 LINE: while (<STDIN>) {
332 next LINE if /^#/; # discard comments
336 The C<last> command immediately exits the loop in question. The
337 C<continue> block, if any, is not executed:
339 LINE: while (<STDIN>) {
340 last LINE if /^$/; # exit when done with header
344 The C<redo> command restarts the loop block without evaluating the
345 conditional again. The C<continue> block, if any, is I<not> executed.
346 This command is normally used by programs that want to lie to themselves
347 about what was just input.
349 For example, when processing a file like F</etc/termcap>.
350 If your input lines might end in backslashes to indicate continuation, you
351 want to skip ahead and get the next record.
362 which is Perl shorthand for the more explicitly written version:
364 LINE: while (defined($line = <ARGV>)) {
366 if ($line =~ s/\\$//) {
368 redo LINE unless eof(); # not eof(ARGV)!
373 Note that if there were a C<continue> block on the above code, it would
374 get executed only on lines discarded by the regex (since redo skips the
375 continue block). A continue block is often used to reset line counters
376 or C<m?pat?> one-time matches:
378 # inspired by :1,$g/fred/s//WILMA/
380 m?(fred)? && s//WILMA $1 WILMA/;
381 m?(barney)? && s//BETTY $1 BETTY/;
382 m?(homer)? && s//MARGE $1 MARGE/;
384 print "$ARGV $.: $_";
385 close ARGV if eof; # reset $.
386 reset if eof; # reset ?pat?
389 If the word C<while> is replaced by the word C<until>, the sense of the
390 test is reversed, but the conditional is still tested before the first
393 Loop control statements don't work in an C<if> or C<unless>, since
394 they aren't loops. You can double the braces to make them such, though.
398 next if /barney/; # same effect as "last",
399 # but doesn't document as well
403 This is caused by the fact that a block by itself acts as a loop that
404 executes once, see L</"Basic BLOCKs">.
406 The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
407 available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
412 Perl's C-style C<for> loop works like the corresponding C<while> loop;
413 that means that this:
415 for ($i = 1; $i < 10; $i++) {
428 There is one minor difference: if variables are declared with C<my>
429 in the initialization section of the C<for>, the lexical scope of
430 those variables is exactly the C<for> loop (the body of the loop
431 and the control sections).
434 As a special case, if the test in the C<for> loop (or the corresponding
435 C<while> loop) is empty, it is treated as true. That is, both
447 are treated as infinite loops.
449 Besides the normal array index looping, C<for> can lend itself
450 to many other interesting applications. Here's one that avoids the
451 problem you get into if you explicitly test for end-of-file on
452 an interactive file descriptor causing your program to appear to
454 X<eof> X<end-of-file> X<end of file>
456 $on_a_tty = -t STDIN && -t STDOUT;
457 sub prompt { print "yes? " if $on_a_tty }
458 for ( prompt(); <STDIN>; prompt() ) {
462 Using C<readline> (or the operator form, C<< <EXPR> >>) as the
463 conditional of a C<for> loop is shorthand for the following. This
464 behaviour is the same as a C<while> loop conditional.
465 X<readline> X<< <> >>
467 for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
474 The C<foreach> loop iterates over a normal list value and sets the scalar
475 variable VAR to be each element of the list in turn. If the variable
476 is preceded with the keyword C<my>, then it is lexically scoped, and
477 is therefore visible only within the loop. Otherwise, the variable is
478 implicitly local to the loop and regains its former value upon exiting
479 the loop. If the variable was previously declared with C<my>, it uses
480 that variable instead of the global one, but it's still localized to
481 the loop. This implicit localization occurs I<only> in a C<foreach>
485 The C<foreach> keyword is actually a synonym for the C<for> keyword, so
486 you can use either. If VAR is omitted, C<$_> is set to each value.
489 If any element of LIST is an lvalue, you can modify it by modifying
490 VAR inside the loop. Conversely, if any element of LIST is NOT an
491 lvalue, any attempt to modify that element will fail. In other words,
492 the C<foreach> loop index variable is an implicit alias for each item
493 in the list that you're looping over.
496 If any part of LIST is an array, C<foreach> will get very confused if
497 you add or remove elements within the loop body, for example with
498 C<splice>. So don't do that.
501 C<foreach> probably won't do what you expect if VAR is a tied or other
502 special variable. Don't do that either.
504 As of Perl 5.22, there is an experimental variant of this loop that accepts
505 a variable preceded by a backslash for VAR, in which case the items in the
506 LIST must be references. The backslashed variable will become an alias
507 to each referenced item in the LIST, which must be of the correct type.
508 The variable needn't be a scalar in this case, and the backslash may be
509 followed by C<my>. To use this form, you must enable the C<refaliasing>
510 feature via C<use feature>. (See L<feature>. See also L<perlref/Assigning
515 for (@ary) { s/foo/bar/ }
517 for my $elem (@elements) {
521 for $count (reverse(1..10), "BOOM") {
526 for (1..15) { print "Merry Christmas\n"; }
528 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
529 print "Item: $item\n";
532 use feature "refaliasing";
533 no warnings "experimental::refaliasing";
534 foreach \my %hash (@array_of_hash_references) {
535 # do something which each %hash
538 Here's how a C programmer might code up a particular algorithm in Perl:
540 for (my $i = 0; $i < @ary1; $i++) {
541 for (my $j = 0; $j < @ary2; $j++) {
542 if ($ary1[$i] > $ary2[$j]) {
543 last; # can't go to outer :-(
545 $ary1[$i] += $ary2[$j];
547 # this is where that last takes me
550 Whereas here's how a Perl programmer more comfortable with the idiom might
553 OUTER: for my $wid (@ary1) {
554 INNER: for my $jet (@ary2) {
555 next OUTER if $wid > $jet;
560 See how much easier this is? It's cleaner, safer, and faster. It's
561 cleaner because it's less noisy. It's safer because if code gets added
562 between the inner and outer loops later on, the new code won't be
563 accidentally executed. The C<next> explicitly iterates the other loop
564 rather than merely terminating the inner one. And it's faster because
565 Perl executes a C<foreach> statement more rapidly than it would the
566 equivalent C<for> loop.
568 Perceptive Perl hackers may have noticed that a C<for> loop has a return
569 value, and that this value can be captured by wrapping the loop in a C<do>
570 block. The reward for this discovery is this cautionary advice: The
571 return value of a C<for> loop is unspecified and may change without notice.
577 A BLOCK by itself (labeled or not) is semantically equivalent to a
578 loop that executes once. Thus you can use any of the loop control
579 statements in it to leave or restart the block. (Note that this is
580 I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
581 C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
584 The BLOCK construct can be used to emulate case structures.
587 if (/^abc/) { $abc = 1; last SWITCH; }
588 if (/^def/) { $def = 1; last SWITCH; }
589 if (/^xyz/) { $xyz = 1; last SWITCH; }
593 You'll also find that C<foreach> loop used to create a topicalizer
598 if (/^abc/) { $abc = 1; last SWITCH; }
599 if (/^def/) { $def = 1; last SWITCH; }
600 if (/^xyz/) { $xyz = 1; last SWITCH; }
604 Such constructs are quite frequently used, both because older versions of
605 Perl had no official C<switch> statement, and also because the new version
606 described immediately below remains experimental and can sometimes be confusing.
608 =head2 Switch Statements
610 X<switch> X<case> X<given> X<when> X<default>
612 Starting from Perl 5.10.1 (well, 5.10.0, but it didn't work
615 use feature "switch";
617 to enable an experimental switch feature. This is loosely based on an
618 old version of a Perl 6 proposal, but it no longer resembles the Perl 6
619 construct. You also get the switch feature whenever you declare that your
620 code prefers to run under a version of Perl that is 5.10 or later. For
625 Under the "switch" feature, Perl gains the experimental keywords
626 C<given>, C<when>, C<default>, C<continue>, and C<break>.
627 Starting from Perl 5.16, one can prefix the switch
628 keywords with C<CORE::> to access the feature without a C<use feature>
629 statement. The keywords C<given> and
630 C<when> are analogous to C<switch> and
631 C<case> in other languages -- though C<continue> is not -- so the code
632 in the previous section could be rewritten as
636 when (/^abc/) { $abc = 1 }
637 when (/^def/) { $def = 1 }
638 when (/^xyz/) { $xyz = 1 }
639 default { $nothing = 1 }
642 The C<foreach> is the non-experimental way to set a topicalizer.
643 If you wish to use the highly experimental C<given>, that could be
648 when (/^abc/) { $abc = 1 }
649 when (/^def/) { $def = 1 }
650 when (/^xyz/) { $xyz = 1 }
651 default { $nothing = 1 }
654 As of 5.14, that can also be written this way:
658 $abc = 1 when /^abc/;
659 $def = 1 when /^def/;
660 $xyz = 1 when /^xyz/;
661 default { $nothing = 1 }
664 Or if you don't care to play it safe, like this:
668 $abc = 1 when /^abc/;
669 $def = 1 when /^def/;
670 $xyz = 1 when /^xyz/;
671 default { $nothing = 1 }
674 The arguments to C<given> and C<when> are in scalar context,
675 and C<given> assigns the C<$_> variable its topic value.
677 Exactly what the I<EXPR> argument to C<when> does is hard to describe
678 precisely, but in general, it tries to guess what you want done. Sometimes
679 it is interpreted as C<< $_ ~~ I<EXPR> >>, and sometimes it is not. It
680 also behaves differently when lexically enclosed by a C<given> block than
681 it does when dynamically enclosed by a C<foreach> loop. The rules are far
682 too difficult to understand to be described here. See L</"Experimental Details
683 on given and when"> later on.
685 Due to an unfortunate bug in how C<given> was implemented between Perl 5.10
686 and 5.16, under those implementations the version of C<$_> governed by
687 C<given> is merely a lexically scoped copy of the original, not a
688 dynamically scoped alias to the original, as it would be if it were a
689 C<foreach> or under both the original and the current Perl 6 language
690 specification. This bug was fixed in Perl 5.18 (and lexicalized C<$_> itself
691 was removed in Perl 5.24).
693 If your code still needs to run on older versions,
694 stick to C<foreach> for your topicalizer and
695 you will be less unhappy.
700 Although not for the faint of heart, Perl does support a C<goto>
701 statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and
702 C<goto>-&NAME. A loop's LABEL is not actually a valid target for
703 a C<goto>; it's just the name of the loop.
705 The C<goto>-LABEL form finds the statement labeled with LABEL and resumes
706 execution there. It may not be used to go into any construct that
707 requires initialization, such as a subroutine or a C<foreach> loop. It
708 also can't be used to go into a construct that is optimized away. It
709 can be used to go almost anywhere else within the dynamic scope,
710 including out of subroutines, but it's usually better to use some other
711 construct such as C<last> or C<die>. The author of Perl has never felt the
712 need to use this form of C<goto> (in Perl, that is--C is another matter).
714 The C<goto>-EXPR form expects a label name, whose scope will be resolved
715 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
716 necessarily recommended if you're optimizing for maintainability:
718 goto(("FOO", "BAR", "GLARCH")[$i]);
720 The C<goto>-&NAME form is highly magical, and substitutes a call to the
721 named subroutine for the currently running subroutine. This is used by
722 C<AUTOLOAD()> subroutines that wish to load another subroutine and then
723 pretend that the other subroutine had been called in the first place
724 (except that any modifications to C<@_> in the current subroutine are
725 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
726 will be able to tell that this routine was called first.
728 In almost all cases like this, it's usually a far, far better idea to use the
729 structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
730 resorting to a C<goto>. For certain applications, the catch and throw pair of
731 C<eval{}> and die() for exception processing can also be a prudent approach.
733 =head2 The Ellipsis Statement
737 X<elliptical statement>
738 X<unimplemented statement>
739 X<unimplemented operator>
741 X<yada-yada operator>
744 X<triple-dot operator>
746 Beginning in Perl 5.12, Perl accepts an ellipsis, "C<...>", as a
747 placeholder for code that you haven't implemented yet.
748 When Perl 5.12 or later encounters an ellipsis statement, it parses this
749 without error, but if and when you should actually try to execute it, Perl
750 throws an exception with the text C<Unimplemented>:
753 sub unimplemented { ... }
754 eval { unimplemented() };
755 if ($@ =~ /^Unimplemented at /) {
756 say "I found an ellipsis!";
759 You can only use the elliptical statement to stand in for a complete
760 statement. Syntactically, "C<...;>" is a complete statement, but,
761 as with other kinds of semicolon-terminated statement, the semicolon
762 may be omitted if "C<...>" appears immediately before a closing brace.
763 These examples show how the ellipsis works:
781 The elliptical statement cannot stand in for an expression that
782 is part of a larger statement.
783 These examples of attempts to use an ellipsis are syntax errors:
788 open(my $fh, ">", "/dev/passwd") or ...;
789 if ($condition && ... ) { say "Howdy" };
791 say "Cromulent" if ...;
794 There are some cases where Perl can't immediately tell the difference
795 between an expression and a statement. For instance, the syntax for a
796 block and an anonymous hash reference constructor look the same unless
797 there's something in the braces to give Perl a hint. The ellipsis is a
798 syntax error if Perl doesn't guess that the C<{ ... }> is a block.
799 Inside your block, you can use a C<;> before the ellipsis to denote that the
800 C<{ ... }> is a block and not a hash reference constructor.
802 Note: Some folks colloquially refer to this bit of punctuation as a
803 "yada-yada" or "triple-dot", but its true name
804 is actually an ellipsis.
806 =head2 PODs: Embedded Documentation
807 X<POD> X<documentation>
809 Perl has a mechanism for intermixing documentation with source code.
810 While it's expecting the beginning of a new statement, if the compiler
811 encounters a line that begins with an equal sign and a word, like this
813 =head1 Here There Be Pods!
815 Then that text and all remaining text up through and including a line
816 beginning with C<=cut> will be ignored. The format of the intervening
817 text is described in L<perlpod>.
819 This allows you to intermix your source code
820 and your documentation text freely, as in
824 The snazzle() function will behave in the most spectacular
825 form that you can possibly imagine, not even excepting
826 cybernetic pyrotechnics.
828 =cut back to the compiler, nuff of this pod stuff!
835 Note that pod translators should look at only paragraphs beginning
836 with a pod directive (it makes parsing easier), whereas the compiler
837 actually knows to look for pod escapes even in the middle of a
838 paragraph. This means that the following secret stuff will be
839 ignored by both the compiler and the translators.
843 warn "Neither POD nor CODE!?"
847 You probably shouldn't rely upon the C<warn()> being podded out forever.
848 Not all pod translators are well-behaved in this regard, and perhaps
849 the compiler will become pickier.
851 One may also use pod directives to quickly comment out a section
854 =head2 Plain Old Comments (Not!)
855 X<comment> X<line> X<#> X<preprocessor> X<eval>
857 Perl can process line directives, much like the C preprocessor. Using
858 this, one can control Perl's idea of filenames and line numbers in
859 error or warning messages (especially for strings that are processed
860 with C<eval()>). The syntax for this mechanism is almost the same as for
861 most C preprocessors: it matches the regular expression
863 # example: '# line 42 "new_filename.plx"'
866 (?:\s("?)([^"]+)\g2)? \s*
869 with C<$1> being the line number for the next line, and C<$3> being
870 the optional filename (specified with or without quotes). Note that
871 no whitespace may precede the C<< # >>, unlike modern C preprocessors.
873 There is a fairly obvious gotcha included with the line directive:
874 Debuggers and profilers will only show the last source line to appear
875 at a particular line number in a given file. Care should be taken not
876 to cause line number collisions in code you'd like to debug later.
878 Here are some examples that you should be able to type into your command
883 # the '#' on the previous line must be the first char on line
886 foo at bzzzt line 201.
890 eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
895 eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
897 foo at foo bar line 200.
901 eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
904 foo at goop line 345.
906 =head2 Experimental Details on given and when
908 As previously mentioned, the "switch" feature is considered highly
909 experimental; it is subject to change with little notice. In particular,
910 C<when> has tricky behaviours that are expected to change to become less
911 tricky in the future. Do not rely upon its current (mis)implementation.
912 Before Perl 5.18, C<given> also had tricky behaviours that you should still
913 beware of if your code must run on older versions of Perl.
915 Here is a longer example of C<given>:
920 say '$foo is undefined';
923 say '$foo is the string "foo"';
926 say '$foo is an odd digit';
927 continue; # Fall through
930 say '$foo is numerically less than 100';
932 when (\&complicated_check) {
933 say 'a complicated check for $foo is true';
936 die q(I don't know what to do with $foo);
940 Before Perl 5.18, C<given(EXPR)> assigned the value of I<EXPR> to
941 merely a lexically scoped I<B<copy>> (!) of C<$_>, not a dynamically
942 scoped alias the way C<foreach> does. That made it similar to
944 do { my $_ = EXPR; ... }
946 except that the block was automatically broken out of by a successful
947 C<when> or an explicit C<break>. Because it was only a copy, and because
948 it was only lexically scoped, not dynamically scoped, you could not do the
949 things with it that you are used to in a C<foreach> loop. In particular,
950 it did not work for arbitrary function calls if those functions might try
951 to access $_. Best stick to C<foreach> for that.
953 Most of the power comes from the implicit smartmatching that can
954 sometimes apply. Most of the time, C<when(EXPR)> is treated as an
955 implicit smartmatch of C<$_>, that is, C<$_ ~~ EXPR>. (See
956 L<perlop/"Smartmatch Operator"> for more information on smartmatching.)
957 But when I<EXPR> is one of the 10 exceptional cases (or things like them)
958 listed below, it is used directly as a boolean.
964 A user-defined subroutine call or a method invocation.
968 A regular expression match in the form of C</REGEX/>, C<$foo =~ /REGEX/>,
969 or C<$foo =~ EXPR>. Also, a negated regular expression match in
970 the form C<!/REGEX/>, C<$foo !~ /REGEX/>, or C<$foo !~ EXPR>.
974 A smart match that uses an explicit C<~~> operator, such as C<EXPR ~~ EXPR>.
976 B<NOTE:> You will often have to use C<$c ~~ $_> because the default case
977 uses C<$_ ~~ $c> , which is frequentlythe opposite of what you want.
981 A boolean comparison operator such as C<$_ E<lt> 10> or C<$x eq "abc">. The
982 relational operators that this applies to are the six numeric comparisons
983 (C<< < >>, C<< > >>, C<< <= >>, C<< >= >>, C<< == >>, and C<< != >>), and
984 the six string comparisons (C<lt>, C<gt>, C<le>, C<ge>, C<eq>, and C<ne>).
988 At least the three builtin functions C<defined(...)>, C<exists(...)>, and
989 C<eof(...)>. We might someday add more of these later if we think of them.
993 A negated expression, whether C<!(EXPR)> or C<not(EXPR)>, or a logical
994 exclusive-or, C<(EXPR1) xor (EXPR2)>. The bitwise versions (C<~> and C<^>)
999 A filetest operator, with exactly 4 exceptions: C<-s>, C<-M>, C<-A>, and
1000 C<-C>, as these return numerical values, not boolean ones. The C<-z>
1001 filetest operator is not included in the exception list.
1005 The C<..> and C<...> flip-flop operators. Note that the C<...> flip-flop
1006 operator is completely different from the C<...> elliptical statement
1011 In those 8 cases above, the value of EXPR is used directly as a boolean, so
1012 no smartmatching is done. You may think of C<when> as a smartsmartmatch.
1014 Furthermore, Perl inspects the operands of logical operators to
1015 decide whether to use smartmatching for each one by applying the
1016 above test to the operands:
1022 If EXPR is C<EXPR1 && EXPR2> or C<EXPR1 and EXPR2>, the test is applied
1023 I<recursively> to both EXPR1 and EXPR2.
1024 Only if I<both> operands also pass the
1025 test, I<recursively>, will the expression be treated as boolean. Otherwise,
1026 smartmatching is used.
1030 If EXPR is C<EXPR1 || EXPR2>, C<EXPR1 // EXPR2>, or C<EXPR1 or EXPR2>, the
1031 test is applied I<recursively> to EXPR1 only (which might itself be a
1032 higher-precedence AND operator, for example, and thus subject to the
1033 previous rule), not to EXPR2. If EXPR1 is to use smartmatching, then EXPR2
1034 also does so, no matter what EXPR2 contains. But if EXPR2 does not get to
1035 use smartmatching, then the second argument will not be either. This is
1036 quite different from the C<&&> case just described, so be careful.
1040 These rules are complicated, but the goal is for them to do what you want
1041 (even if you don't quite understand why they are doing it). For example:
1043 when (/^\d+$/ && $_ < 75) { ... }
1045 will be treated as a boolean match because the rules say both
1046 a regex match and an explicit test on C<$_> will be treated
1051 when ([qw(foo bar)] && /baz/) { ... }
1053 will use smartmatching because only I<one> of the operands is a boolean:
1054 the other uses smartmatching, and that wins.
1058 when ([qw(foo bar)] || /^baz/) { ... }
1060 will use smart matching (only the first operand is considered), whereas
1062 when (/^baz/ || [qw(foo bar)]) { ... }
1064 will test only the regex, which causes both operands to be
1065 treated as boolean. Watch out for this one, then, because an
1066 arrayref is always a true value, which makes it effectively
1067 redundant. Not a good idea.
1069 Tautologous boolean operators are still going to be optimized
1070 away. Don't be tempted to write
1072 when ("foo" or "bar") { ... }
1074 This will optimize down to C<"foo">, so C<"bar"> will never be considered (even
1075 though the rules say to use a smartmatch
1076 on C<"foo">). For an alternation like
1077 this, an array ref will work, because this will instigate smartmatching:
1079 when ([qw(foo bar)] { ... }
1081 This is somewhat equivalent to the C-style switch statement's fallthrough
1082 functionality (not to be confused with I<Perl's> fallthrough
1083 functionality--see below), wherein the same block is used for several
1086 Another useful shortcut is that, if you use a literal array or hash as the
1087 argument to C<given>, it is turned into a reference. So C<given(@foo)> is
1088 the same as C<given(\@foo)>, for example.
1090 C<default> behaves exactly like C<when(1 == 1)>, which is
1091 to say that it always matches.
1095 You can use the C<break> keyword to break out of the enclosing
1096 C<given> block. Every C<when> block is implicitly ended with
1101 You can use the C<continue> keyword to fall through from one
1102 case to the next immediate C<when> or C<default>:
1105 when (/x/) { say '$foo contains an x'; continue }
1106 when (/y/) { say '$foo contains a y' }
1107 default { say '$foo does not contain a y' }
1112 When a C<given> statement is also a valid expression (for example,
1113 when it's the last statement of a block), it evaluates to:
1119 An empty list as soon as an explicit C<break> is encountered.
1123 The value of the last evaluated expression of the successful
1124 C<when>/C<default> clause, if there happens to be one.
1128 The value of the last evaluated expression of the C<given> block if no
1133 In both last cases, the last expression is evaluated in the context that
1134 was applied to the C<given> block.
1136 Note that, unlike C<if> and C<unless>, failed C<when> statements always
1137 evaluate to an empty list.
1141 when (["pear", "apple"]) { 1 }
1142 break when "vote"; # My vote cannot be bought
1143 1e10 when /Mona Lisa/;
1148 Currently, C<given> blocks can't always
1149 be used as proper expressions. This
1150 may be addressed in a future version of Perl.
1152 =head3 Switching in a loop
1154 Instead of using C<given()>, you can use a C<foreach()> loop.
1155 For example, here's one way to count how many times a particular
1156 string occurs in an array:
1161 when ("foo") { ++$count }
1163 print "\@array contains $count copies of 'foo'\n";
1165 Or in a more recent version:
1170 ++$count when "foo";
1172 print "\@array contains $count copies of 'foo'\n";
1174 At the end of all C<when> blocks, there is an implicit C<next>.
1175 You can override that with an explicit C<last> if you're
1176 interested in only the first match alone.
1178 This doesn't work if you explicitly specify a loop variable, as
1179 in C<for $item (@array)>. You have to use the default variable C<$_>.
1181 =head3 Differences from Perl 6
1183 The Perl 5 smartmatch and C<given>/C<when> constructs are not compatible
1184 with their Perl 6 analogues. The most visible difference and least
1185 important difference is that, in Perl 5, parentheses are required around
1186 the argument to C<given()> and C<when()> (except when this last one is used
1187 as a statement modifier). Parentheses in Perl 6 are always optional in a
1188 control construct such as C<if()>, C<while()>, or C<when()>; they can't be
1189 made optional in Perl 5 without a great deal of potential confusion,
1190 because Perl 5 would parse the expression
1196 as though the argument to C<given> were an element of the hash
1197 C<%foo>, interpreting the braces as hash-element syntax.
1199 However, their are many, many other differences. For example,
1200 this works in Perl 5:
1203 my @primary = ("red", "blue", "green");
1205 if (@primary ~~ "red") {
1206 say "primary smartmatches red";
1209 if ("red" ~~ @primary) {
1210 say "red smartmatches primary";
1213 say "that's all, folks!";
1215 But it doesn't work at all in Perl 6. Instead, you should
1216 use the (parallelizable) C<any> operator:
1218 if any(@primary) eq "red" {
1219 say "primary smartmatches red";
1222 if "red" eq any(@primary) {
1223 say "red smartmatches primary";
1226 The table of smartmatches in L<perlop/"Smartmatch Operator"> is not
1227 identical to that proposed by the Perl 6 specification, mainly due to
1228 differences between Perl 6's and Perl 5's data models, but also because
1229 the Perl 6 spec has changed since Perl 5 rushed into early adoption.
1231 In Perl 6, C<when()> will always do an implicit smartmatch with its
1232 argument, while in Perl 5 it is convenient (albeit potentially confusing) to
1233 suppress this implicit smartmatch in various rather loosely-defined
1234 situations, as roughly outlined above. (The difference is largely because
1235 Perl 5 does not have, even internally, a boolean type.)