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 mostly serves to separate tokens, unlike
14 languages like Python where it is an important part of the syntax.
16 Many of Perl's syntactic elements are B<optional>. Rather than
17 requiring you to put parentheses around every function call and
18 declare every variable, you can often leave such explicit elements off
19 and Perl will figure out what you meant. This is known as B<Do What I
20 Mean>, abbreviated B<DWIM>. It allows programmers to be B<lazy> and to
21 code in a style with which they are comfortable.
23 Perl B<borrows syntax> and concepts from many languages: awk, sed, C,
24 Bourne Shell, Smalltalk, Lisp and even English. Other
25 languages have borrowed syntax from Perl, particularly its regular
26 expression extensions. So if you have programmed in another language
27 you will see familiar pieces in Perl. They often work the same, but
28 see L<perltrap> for information about how they differ.
31 X<declaration> X<undef> X<undefined> X<uninitialized>
33 The only things you need to declare in Perl are report formats and
34 subroutines (and sometimes not even subroutines). A variable holds
35 the undefined value (C<undef>) until it has been assigned a defined
36 value, which is anything other than C<undef>. When used as a number,
37 C<undef> is treated as C<0>; when used as a string, it is treated as
38 the empty string, C<"">; and when used as a reference that isn't being
39 assigned to, it is treated as an error. If you enable warnings,
40 you'll be notified of an uninitialized value whenever you treat
41 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 left values 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. Typically all the declarations are 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()>, you'll
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 $0 or die "can't get myname";
73 Note that myname() functions as a list operator, not as a unary operator;
74 so be careful to use C<or> instead of C<||> in this case. However, if
75 you were to declare the subroutine as C<sub myname ($)>, then
76 C<myname> would function as a unary operator, so either C<or> or
79 Subroutines declarations can also be loaded up with the C<require> statement
80 or both loaded and imported into your namespace with a C<use> statement.
81 See L<perlmod> for details on this.
83 A statement sequence may contain declarations of lexically-scoped
84 variables, but apart from declaring a variable name, the declaration acts
85 like an ordinary statement, and is elaborated within the sequence of
86 statements as if it were an ordinary statement. That means it actually
87 has both compile-time and run-time effects.
92 Text from a C<"#"> character until the end of the line is a comment,
93 and is ignored. Exceptions include C<"#"> inside a string or regular
96 =head2 Simple Statements
97 X<statement> X<semicolon> X<expression> X<;>
99 The only kind of simple statement is an expression evaluated for its
100 side effects. Every simple statement must be terminated with a
101 semicolon, unless it is the final statement in a block, in which case
102 the semicolon is optional. (A semicolon is still encouraged if the
103 block takes up more than one line, because you may eventually add
104 another line.) Note that there are some operators like C<eval {}> and
105 C<do {}> that look like compound statements, but aren't (they're just
106 TERMs in an expression), and thus need an explicit termination if used
107 as the last item in a statement.
109 =head2 Truth and Falsehood
110 X<truth> X<falsehood> X<true> X<false> X<!> X<not> X<negation> X<0>
112 The number 0, the strings C<'0'> and C<''>, the empty list C<()>, and
113 C<undef> are all false in a boolean context. All other values are true.
114 Negation of a true value by C<!> or C<not> returns a special false value.
115 When evaluated as a string it is treated as C<''>, but as a number, it
118 =head2 Statement Modifiers
119 X<statement modifier> X<modifier> X<if> X<unless> X<while>
120 X<until> X<when> X<foreach> X<for>
122 Any simple statement may optionally be followed by a I<SINGLE> modifier,
123 just before the terminating semicolon (or block ending). The possible
134 The C<EXPR> following the modifier is referred to as the "condition".
135 Its truth or falsehood determines how the modifier will behave.
137 C<if> executes the statement once I<if> and only if the condition is
138 true. C<unless> is the opposite, it executes the statement I<unless>
139 the condition is true (i.e., if the condition is false).
141 print "Basset hounds got long ears" if length $ear >= 10;
142 go_outside() and play() unless $is_raining;
144 C<when> executes the statement I<when> C<$_> smart matches C<EXPR>, and
145 then either C<break>s out if it's enclosed in a C<given> scope or skips
146 to the C<next> element when it lies directly inside a C<for> loop.
147 See also L</"Switch statements">.
150 $abc = 1 when /^abc/;
151 $just_a = 1 when /^a/;
156 admin($_) when [ qw/Alice Bob/ ];
157 regular($_) when [ qw/Chris David Ellen/ ];
160 The C<foreach> modifier is an iterator: it executes the statement once
161 for each item in the LIST (with C<$_> aliased to each item in turn).
163 print "Hello $_!\n" foreach qw(world Dolly nurse);
165 C<while> repeats the statement I<while> the condition is true.
166 C<until> does the opposite, it repeats the statement I<until> the
167 condition is true (or while the condition is false):
169 # Both of these count from 0 to 10.
170 print $i++ while $i <= 10;
171 print $j++ until $j > 10;
173 The C<while> and C<until> modifiers have the usual "C<while> loop"
174 semantics (conditional evaluated first), except when applied to a
175 C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE statement), in
176 which case the block executes once before the conditional is
177 evaluated. This is so that you can write loops like:
182 } until $line eq ".\n";
184 See L<perlfunc/do>. Note also that the loop control statements described
185 later will I<NOT> work in this construct, because modifiers don't take
186 loop labels. Sorry. You can always put another block inside of it
187 (for C<next>) or around it (for C<last>) to do that sort of thing.
188 For C<next>, just double the braces:
189 X<next> X<last> X<redo>
196 For C<last>, you have to be more elaborate:
206 B<NOTE:> The behaviour of a C<my> statement modified with a statement
207 modifier conditional or loop construct (e.g. 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 =head2 Compound Statements
215 X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace>
216 X<{> X<}> X<if> X<unless> X<while> X<until> X<foreach> X<for> X<continue>
218 In Perl, a sequence of statements that defines a scope is called a block.
219 Sometimes a block is delimited by the file containing it (in the case
220 of a required file, or the program as a whole), and sometimes a block
221 is delimited by the extent of a string (in the case of an eval).
223 But generally, a block is delimited by curly brackets, also known as braces.
224 We will call this syntactic construct a BLOCK.
226 The following compound statements may be used to control flow:
229 if (EXPR) BLOCK else BLOCK
230 if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
232 unless (EXPR) BLOCK else BLOCK
233 unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
234 LABEL while (EXPR) BLOCK
235 LABEL while (EXPR) BLOCK continue BLOCK
236 LABEL until (EXPR) BLOCK
237 LABEL until (EXPR) BLOCK continue BLOCK
238 LABEL for (EXPR; EXPR; EXPR) BLOCK
239 LABEL foreach VAR (LIST) BLOCK
240 LABEL foreach VAR (LIST) BLOCK continue BLOCK
241 LABEL BLOCK continue BLOCK
243 Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
244 not statements. This means that the curly brackets are I<required>--no
245 dangling statements allowed. If you want to write conditionals without
246 curly brackets there are several other ways to do it. The following
247 all do the same thing:
249 if (!open(FOO)) { die "Can't open $FOO: $!"; }
250 die "Can't open $FOO: $!" unless open(FOO);
251 open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
252 open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
253 # a bit exotic, that last one
255 The C<if> statement is straightforward. Because BLOCKs are always
256 bounded by curly brackets, there is never any ambiguity about which
257 C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
258 the sense of the test is reversed. Like C<if>, C<unless> can be followed
259 by C<else>. C<unless> can even be followed by one or more C<elsif>
260 statements, though you may want to think twice before using that particular
261 language construct, as everyone reading your code will have to think at least
262 twice before they can understand what's going on.
264 The C<while> statement executes the block as long as the expression is
265 L<true|/"Truth and Falsehood">.
266 The C<until> statement executes the block as long as the expression is
268 The LABEL is optional, and if present, consists of an identifier followed
269 by a colon. The LABEL identifies the loop for the loop control
270 statements C<next>, C<last>, and C<redo>.
271 If the LABEL is omitted, the loop control statement
272 refers to the innermost enclosing loop. This may include dynamically
273 looking back your call-stack at run time to find the LABEL. Such
274 desperate behavior triggers a warning if you use the C<use warnings>
275 pragma or the B<-w> flag.
277 If there is a C<continue> BLOCK, it is always executed just before the
278 conditional is about to be evaluated again. Thus it can be used to
279 increment a loop variable, even when the loop has been continued via
280 the C<next> statement.
282 Extension modules can also hook into the Perl parser to define new
283 kinds of compound statement. These are introduced by a keyword which
284 the extension recognizes, and the syntax following the keyword is
285 defined entirely by the extension. If you are an implementor, see
286 L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
287 a module, see the module's documentation for details of the syntax that
291 X<loop control> X<loop, control> X<next> X<last> X<redo> X<continue>
293 The C<next> command starts the next iteration of the loop:
295 LINE: while (<STDIN>) {
296 next LINE if /^#/; # discard comments
300 The C<last> command immediately exits the loop in question. The
301 C<continue> block, if any, is not executed:
303 LINE: while (<STDIN>) {
304 last LINE if /^$/; # exit when done with header
308 The C<redo> command restarts the loop block without evaluating the
309 conditional again. The C<continue> block, if any, is I<not> executed.
310 This command is normally used by programs that want to lie to themselves
311 about what was just input.
313 For example, when processing a file like F</etc/termcap>.
314 If your input lines might end in backslashes to indicate continuation, you
315 want to skip ahead and get the next record.
326 which is Perl short-hand for the more explicitly written version:
328 LINE: while (defined($line = <ARGV>)) {
330 if ($line =~ s/\\$//) {
332 redo LINE unless eof(); # not eof(ARGV)!
337 Note that if there were a C<continue> block on the above code, it would
338 get executed only on lines discarded by the regex (since redo skips the
339 continue block). A continue block is often used to reset line counters
340 or C<m?pat?> one-time matches:
342 # inspired by :1,$g/fred/s//WILMA/
344 m?(fred)? && s//WILMA $1 WILMA/;
345 m?(barney)? && s//BETTY $1 BETTY/;
346 m?(homer)? && s//MARGE $1 MARGE/;
348 print "$ARGV $.: $_";
349 close ARGV if eof; # reset $.
350 reset if eof; # reset ?pat?
353 If the word C<while> is replaced by the word C<until>, the sense of the
354 test is reversed, but the conditional is still tested before the first
357 The loop control statements don't work in an C<if> or C<unless>, since
358 they aren't loops. You can double the braces to make them such, though.
362 next if /barney/; # same effect as "last", but doesn't document as well
366 This is caused by the fact that a block by itself acts as a loop that
367 executes once, see L<"Basic BLOCKs">.
369 The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
370 available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
375 Perl's C-style C<for> loop works like the corresponding C<while> loop;
376 that means that this:
378 for ($i = 1; $i < 10; $i++) {
391 There is one minor difference: if variables are declared with C<my>
392 in the initialization section of the C<for>, the lexical scope of
393 those variables is exactly the C<for> loop (the body of the loop
394 and the control sections).
397 Besides the normal array index looping, C<for> can lend itself
398 to many other interesting applications. Here's one that avoids the
399 problem you get into if you explicitly test for end-of-file on
400 an interactive file descriptor causing your program to appear to
402 X<eof> X<end-of-file> X<end of file>
404 $on_a_tty = -t STDIN && -t STDOUT;
405 sub prompt { print "yes? " if $on_a_tty }
406 for ( prompt(); <STDIN>; prompt() ) {
410 Using C<readline> (or the operator form, C<< <EXPR> >>) as the
411 conditional of a C<for> loop is shorthand for the following. This
412 behaviour is the same as a C<while> loop conditional.
413 X<readline> X<< <> >>
415 for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
422 The C<foreach> loop iterates over a normal list value and sets the
423 variable VAR to be each element of the list in turn. If the variable
424 is preceded with the keyword C<my>, then it is lexically scoped, and
425 is therefore visible only within the loop. Otherwise, the variable is
426 implicitly local to the loop and regains its former value upon exiting
427 the loop. If the variable was previously declared with C<my>, it uses
428 that variable instead of the global one, but it's still localized to
429 the loop. This implicit localization occurs I<only> in a C<foreach>
433 The C<foreach> keyword is actually a synonym for the C<for> keyword, so
434 you can use C<foreach> for readability or C<for> for brevity. (Or because
435 the Bourne shell is more familiar to you than I<csh>, so writing C<for>
436 comes more naturally.) If VAR is omitted, C<$_> is set to each value.
439 If any element of LIST is an lvalue, you can modify it by modifying
440 VAR inside the loop. Conversely, if any element of LIST is NOT an
441 lvalue, any attempt to modify that element will fail. In other words,
442 the C<foreach> loop index variable is an implicit alias for each item
443 in the list that you're looping over.
446 If any part of LIST is an array, C<foreach> will get very confused if
447 you add or remove elements within the loop body, for example with
448 C<splice>. So don't do that.
451 C<foreach> probably won't do what you expect if VAR is a tied or other
452 special variable. Don't do that either.
456 for (@ary) { s/foo/bar/ }
458 for my $elem (@elements) {
462 for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
463 print $count, "\n"; sleep(1);
466 for (1..15) { print "Merry Christmas\n"; }
468 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
469 print "Item: $item\n";
472 Here's how a C programmer might code up a particular algorithm in Perl:
474 for (my $i = 0; $i < @ary1; $i++) {
475 for (my $j = 0; $j < @ary2; $j++) {
476 if ($ary1[$i] > $ary2[$j]) {
477 last; # can't go to outer :-(
479 $ary1[$i] += $ary2[$j];
481 # this is where that last takes me
484 Whereas here's how a Perl programmer more comfortable with the idiom might
487 OUTER: for my $wid (@ary1) {
488 INNER: for my $jet (@ary2) {
489 next OUTER if $wid > $jet;
494 See how much easier this is? It's cleaner, safer, and faster. It's
495 cleaner because it's less noisy. It's safer because if code gets added
496 between the inner and outer loops later on, the new code won't be
497 accidentally executed. The C<next> explicitly iterates the other loop
498 rather than merely terminating the inner one. And it's faster because
499 Perl executes a C<foreach> statement more rapidly than it would the
500 equivalent C<for> loop.
505 A BLOCK by itself (labeled or not) is semantically equivalent to a
506 loop that executes once. Thus you can use any of the loop control
507 statements in it to leave or restart the block. (Note that this is
508 I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
509 C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
512 The BLOCK construct can be used to emulate case structures.
515 if (/^abc/) { $abc = 1; last SWITCH; }
516 if (/^def/) { $def = 1; last SWITCH; }
517 if (/^xyz/) { $xyz = 1; last SWITCH; }
521 Such constructs are quite frequently used, because older versions
522 of Perl had no official C<switch> statement.
524 =head2 Switch statements
526 X<switch> X<case> X<given> X<when> X<default>
528 Starting from Perl 5.10, you can say
530 use feature "switch";
532 which enables a switch feature that is closely based on the
535 The keywords C<given> and C<when> are analogous
536 to C<switch> and C<case> in other languages, so the code
537 above could be written as
540 when (/^abc/) { $abc = 1; }
541 when (/^def/) { $def = 1; }
542 when (/^xyz/) { $xyz = 1; }
543 default { $nothing = 1; }
546 This construct is very flexible and powerful. For example:
551 say '$foo is undefined';
554 say '$foo is the string "foo"';
557 say '$foo is an odd digit';
558 continue; # Fall through
561 say '$foo is numerically less than 100';
563 when (\&complicated_check) {
564 say 'a complicated check for $foo is true';
567 die q(I don't know what to do with $foo);
571 C<given(EXPR)> will assign the value of EXPR to C<$_>
572 within the lexical scope of the block, so it's similar to
574 do { my $_ = EXPR; ... }
576 except that the block is automatically broken out of by a
577 successful C<when> or an explicit C<break>.
579 Most of the power comes from implicit smart matching:
583 is exactly equivalent to
587 Most of the time, C<when(EXPR)> is treated as an implicit smart match of
588 C<$_>, i.e. C<$_ ~~ EXPR>. (See L</"Smart matching in detail"> for more
589 information on smart matching.) But when EXPR is one of the below
590 exceptional cases, it is used directly as a boolean:
596 a subroutine or method call
600 a regular expression match, i.e. C</REGEX/> or C<$foo =~ /REGEX/>,
601 or a negated regular expression match (C<!/REGEX/> or C<$foo !~ /REGEX/>).
605 a comparison such as C<$_ E<lt> 10> or C<$x eq "abc">
606 (or of course C<$_ ~~ $c>)
610 C<defined(...)>, C<exists(...)>, or C<eof(...)>
614 a negated expression C<!(...)> or C<not (...)>, or a logical
615 exclusive-or C<(...) xor (...)>.
619 a filetest operator, with the exception of C<-s>, C<-M>, C<-A>, and C<-C>,
620 that return numerical values, not boolean ones.
624 the C<..> and C<...> flip-flop operators.
628 In those cases the value of EXPR is used directly as a boolean.
630 Furthermore, Perl inspects the operands of the binary boolean operators to
631 decide whether to use smart matching for each one by applying the above test to
638 If EXPR is C<... && ...> or C<... and ...>, the test
639 is applied recursively to both operands. If I<both>
640 operands pass the test, then the expression is treated
641 as boolean; otherwise, smart matching is used.
645 If EXPR is C<... || ...>, C<... // ...> or C<... or ...>, the test
646 is applied recursively to the first operand (which may be a
647 higher-precedence AND operator, for example). If the first operand
648 is to use smart matching, then both operands will do so; if it is
649 not, then the second argument will not be either.
653 These rules look complicated, but usually they will do what
654 you want. For example:
656 when (/^\d+$/ && $_ < 75) { ... }
658 will be treated as a boolean match because the rules say both a regex match and
659 an explicit test on $_ will be treated as boolean.
663 when ([qw(foo bar)] && /baz/) { ... }
665 will use smart matching because only I<one> of the operands is a boolean; the
666 other uses smart matching, and that wins.
670 when ([qw(foo bar)] || /^baz/) { ... }
672 will use smart matching (only the first operand is considered), whereas
674 when (/^baz/ || [qw(foo bar)]) { ... }
676 will test only the regex, which causes both operands to be treated as boolean.
677 Watch out for this one, then, because an arrayref is always a true value, which
678 makes it effectively redundant.
680 Tautologous boolean operators are still going to be optimized away. Don't be
683 when ('foo' or 'bar') { ... }
685 This will optimize down to C<'foo'>, so C<'bar'> will never be considered (even
686 though the rules say to use a smart match on C<'foo'>). For an alternation like
687 this, an array ref will work, because this will instigate smart matching:
689 when ([qw(foo bar)] { ... }
691 This is somewhat equivalent to the C-style switch statement's fallthrough
692 functionality (not to be confused with I<Perl's> fallthrough functionality - see
693 below), wherein the same block is used for several C<case> statements.
695 Another useful shortcut is that, if you use a literal array
696 or hash as the argument to C<given>, it is turned into a
697 reference. So C<given(@foo)> is the same as C<given(\@foo)>,
700 C<default> behaves exactly like C<when(1 == 1)>, which is
701 to say that it always matches.
705 You can use the C<break> keyword to break out of the enclosing
706 C<given> block. Every C<when> block is implicitly ended with
711 You can use the C<continue> keyword to fall through from one
715 when (/x/) { say '$foo contains an x'; continue }
716 when (/y/) { say '$foo contains a y' }
717 default { say '$foo does not contain a y' }
722 When a C<given> statement is also a valid expression (e.g.
723 when it's the last statement of a block), it evaluates to :
729 an empty list as soon as an explicit C<break> is encountered.
733 the value of the last evaluated expression of the successful
734 C<when>/C<default> clause, if there's one.
738 the value of the last evaluated expression of the C<given> block if no
743 In both last cases, the last expression is evaluated in the context that
744 was applied to the C<given> block.
746 Note that, unlike C<if> and C<unless>, failed C<when> statements always
747 evaluate to an empty list.
749 my $price = do { given ($item) {
750 when ([ 'pear', 'apple' ]) { 1 }
751 break when 'vote'; # My vote cannot be bought
752 1e10 when /Mona Lisa/;
756 Currently, C<given> blocks can't always be used as proper expressions. This
757 may be addressed in a future version of perl.
759 =head3 Switching in a loop
761 Instead of using C<given()>, you can use a C<foreach()> loop.
762 For example, here's one way to count how many times a particular
763 string occurs in an array:
767 when ("foo") { ++$count }
769 print "\@array contains $count copies of 'foo'\n";
771 At the end of all C<when> blocks, there is an implicit C<next>.
772 You can override that with an explicit C<last> if you're only
773 interested in the first match.
775 This doesn't work if you explicitly specify a loop variable,
776 as in C<for $item (@array)>. You have to use the default
777 variable C<$_>. (You can use C<for my $_ (@array)>.)
779 =head3 Smart matching in detail
781 The behaviour of a smart match depends on what type of thing its arguments
782 are. The behaviour is determined by the following table: the first row
783 that applies determines the match behaviour (which is thus mostly
784 determined by the type of the right operand). Note that the smart match
785 implicitly dereferences any non-blessed hash or array ref, so the "Hash"
786 and "Array" entries apply in those cases. (For blessed references, the
787 "Object" entries apply.)
789 Note that the "Matching Code" column is not always an exact rendition. For
790 example, the smart match operator short-circuits whenever possible, but
793 $a $b Type of Match Implied Matching Code
794 ====== ===== ===================== =============
795 Any undef undefined !defined $a
797 Any Object invokes ~~ overloading on $object, or dies
799 Hash CodeRef sub truth for each key[1] !grep { !$b->($_) } keys %$a
800 Array CodeRef sub truth for each elt[1] !grep { !$b->($_) } @$a
801 Any CodeRef scalar sub truth $b->($a)
803 Hash Hash hash keys identical (every key is found in both hashes)
804 Array Hash hash keys intersection grep { exists $b->{$_} } @$a
805 Regex Hash hash key grep grep /$a/, keys %$b
806 undef Hash always false (undef can't be a key)
807 Any Hash hash entry existence exists $b->{$a}
809 Hash Array hash keys intersection grep { exists $a->{$_} } @$b
810 Array Array arrays are comparable[2]
811 Regex Array array grep grep /$a/, @$b
812 undef Array array contains undef grep !defined, @$b
813 Any Array match against an array element[3]
816 Hash Regex hash key grep grep /$b/, keys %$a
817 Array Regex array grep grep /$b/, @$a
818 Any Regex pattern match $a =~ /$b/
820 Object Any invokes ~~ overloading on $object, or falls back:
821 Any Num numeric equality $a == $b
822 Num numish[4] numeric equality $a == $b
823 undef Any undefined !defined($b)
824 Any Any string equality $a eq $b
826 1 - empty hashes or arrays will match.
827 2 - that is, each element smart-matches the element of same index in the
829 3 - If a circular reference is found, we fall back to referential equality.
830 4 - either a real number, or a string that looks like a number
832 =head3 Custom matching via overloading
834 You can change the way that an object is matched by overloading
835 the C<~~> operator. This may alter the usual smart match semantics.
837 It should be noted that C<~~> will refuse to work on objects that
838 don't overload it (in order to avoid relying on the object's
839 underlying structure).
841 Note also that smart match's matching rules take precedence over
842 overloading, so if C<$obj> has smart match overloading, then
846 will not automatically invoke the overload method with X as an argument;
847 instead the table above is consulted as normal, and based in the type of X,
848 overloading may or may not be invoked.
852 =head3 Differences from Perl 6
854 The Perl 5 smart match and C<given>/C<when> constructs are not
855 absolutely identical to their Perl 6 analogues. The most visible
856 difference is that, in Perl 5, parentheses are required around
857 the argument to C<given()> and C<when()> (except when this last
858 one is used as a statement modifier). Parentheses in Perl 6
859 are always optional in a control construct such as C<if()>,
860 C<while()>, or C<when()>; they can't be made optional in Perl
861 5 without a great deal of potential confusion, because Perl 5
862 would parse the expression
868 as though the argument to C<given> were an element of the hash
869 C<%foo>, interpreting the braces as hash-element syntax.
871 The table of smart matches is not identical to that proposed by the
872 Perl 6 specification, mainly due to the differences between Perl 6's
873 and Perl 5's data models.
875 In Perl 6, C<when()> will always do an implicit smart match
876 with its argument, whilst it is convenient in Perl 5 to
877 suppress this implicit smart match in certain situations,
878 as documented above. (The difference is largely because Perl 5
879 does not, even internally, have a boolean type.)
884 Although not for the faint of heart, Perl does support a C<goto>
885 statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and
886 C<goto>-&NAME. A loop's LABEL is not actually a valid target for
887 a C<goto>; it's just the name of the loop.
889 The C<goto>-LABEL form finds the statement labeled with LABEL and resumes
890 execution there. It may not be used to go into any construct that
891 requires initialization, such as a subroutine or a C<foreach> loop. It
892 also can't be used to go into a construct that is optimized away. It
893 can be used to go almost anywhere else within the dynamic scope,
894 including out of subroutines, but it's usually better to use some other
895 construct such as C<last> or C<die>. The author of Perl has never felt the
896 need to use this form of C<goto> (in Perl, that is--C is another matter).
898 The C<goto>-EXPR form expects a label name, whose scope will be resolved
899 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
900 necessarily recommended if you're optimizing for maintainability:
902 goto(("FOO", "BAR", "GLARCH")[$i]);
904 The C<goto>-&NAME form is highly magical, and substitutes a call to the
905 named subroutine for the currently running subroutine. This is used by
906 C<AUTOLOAD()> subroutines that wish to load another subroutine and then
907 pretend that the other subroutine had been called in the first place
908 (except that any modifications to C<@_> in the current subroutine are
909 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
910 will be able to tell that this routine was called first.
912 In almost all cases like this, it's usually a far, far better idea to use the
913 structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
914 resorting to a C<goto>. For certain applications, the catch and throw pair of
915 C<eval{}> and die() for exception processing can also be a prudent approach.
917 =head2 PODs: Embedded Documentation
918 X<POD> X<documentation>
920 Perl has a mechanism for intermixing documentation with source code.
921 While it's expecting the beginning of a new statement, if the compiler
922 encounters a line that begins with an equal sign and a word, like this
924 =head1 Here There Be Pods!
926 Then that text and all remaining text up through and including a line
927 beginning with C<=cut> will be ignored. The format of the intervening
928 text is described in L<perlpod>.
930 This allows you to intermix your source code
931 and your documentation text freely, as in
935 The snazzle() function will behave in the most spectacular
936 form that you can possibly imagine, not even excepting
937 cybernetic pyrotechnics.
939 =cut back to the compiler, nuff of this pod stuff!
946 Note that pod translators should look at only paragraphs beginning
947 with a pod directive (it makes parsing easier), whereas the compiler
948 actually knows to look for pod escapes even in the middle of a
949 paragraph. This means that the following secret stuff will be
950 ignored by both the compiler and the translators.
954 warn "Neither POD nor CODE!?"
958 You probably shouldn't rely upon the C<warn()> being podded out forever.
959 Not all pod translators are well-behaved in this regard, and perhaps
960 the compiler will become pickier.
962 One may also use pod directives to quickly comment out a section
965 =head2 Plain Old Comments (Not!)
966 X<comment> X<line> X<#> X<preprocessor> X<eval>
968 Perl can process line directives, much like the C preprocessor. Using
969 this, one can control Perl's idea of filenames and line numbers in
970 error or warning messages (especially for strings that are processed
971 with C<eval()>). The syntax for this mechanism is almost the same as for
972 most C preprocessors: it matches the regular expression
974 # example: '# line 42 "new_filename.plx"'
977 (?:\s("?)([^"]+)\g2)? \s*
980 with C<$1> being the line number for the next line, and C<$3> being
981 the optional filename (specified with or without quotes). Note that
982 no whitespace may precede the C<< # >>, unlike modern C preprocessors.
984 There is a fairly obvious gotcha included with the line directive:
985 Debuggers and profilers will only show the last source line to appear
986 at a particular line number in a given file. Care should be taken not
987 to cause line number collisions in code you'd like to debug later.
989 Here are some examples that you should be able to type into your command
994 # the `#' on the previous line must be the first char on line
997 foo at bzzzt line 201.
1001 eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
1006 eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
1008 foo at foo bar line 200.
1012 eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
1015 foo at goop line 345.