7 A Perl script consists of a sequence of declarations and statements.
8 The only things that need to be declared in Perl are report formats
9 and subroutines. See the sections below for more information on those
10 declarations. All uninitialized user-created objects are assumed to
11 start with a null or 0 value until they are defined by some explicit
12 operation such as assignment. (Though you can get warnings about the
13 use of undefined values if you like.) The sequence of statements is
14 executed just once, unlike in B<sed> and B<awk> scripts, where the
15 sequence of statements is executed for each input line. While this means
16 that you must explicitly loop over the lines of your input file (or
17 files), it also means you have much more control over which files and
18 which lines you look at. (Actually, I'm lying--it is possible to do an
19 implicit loop with either the B<-n> or B<-p> switch. It's just not the
20 mandatory default like it is in B<sed> and B<awk>.)
24 Perl is, for the most part, a free-form language. (The only
25 exception to this is format declarations, for obvious reasons.) Comments
26 are indicated by the "#" character, and extend to the end of the line. If
27 you attempt to use C</* */> C-style comments, it will be interpreted
28 either as division or pattern matching, depending on the context, and C++
29 C<//> comments just look like a null regular expression, so don't do
32 A declaration can be put anywhere a statement can, but has no effect on
33 the execution of the primary sequence of statements--declarations all
34 take effect at compile time. Typically all the declarations are put at
35 the beginning or the end of the script. However, if you're using
36 lexically-scoped private variables created with my(), you'll have to make sure
37 your format or subroutine definition is within the same block scope
38 as the my if you expect to to be able to access those private variables.
40 Declaring a subroutine allows a subroutine name to be used as if it were a
41 list operator from that point forward in the program. You can declare a
42 subroutine (prototyped to take one scalar parameter) without defining it by saying just:
45 $me = myname $0 or die "can't get myname";
47 Note that it functions as a list operator though, not as a unary
48 operator, so be careful to use C<or> instead of C<||> there.
50 Subroutines declarations can also be loaded up with the C<require> statement
51 or both loaded and imported into your namespace with a C<use> statement.
52 See L<perlmod> for details on this.
54 A statement sequence may contain declarations of lexically-scoped
55 variables, but apart from declaring a variable name, the declaration acts
56 like an ordinary statement, and is elaborated within the sequence of
57 statements as if it were an ordinary statement. That means it actually
58 has both compile-time and run-time effects.
60 =head2 Simple statements
62 The only kind of simple statement is an expression evaluated for its
63 side effects. Every simple statement must be terminated with a
64 semicolon, unless it is the final statement in a block, in which case
65 the semicolon is optional. (A semicolon is still encouraged there if the
66 block takes up more than one line, since you may eventually add another line.)
67 Note that there are some operators like C<eval {}> and C<do {}> that look
68 like compound statements, but aren't (they're just TERMs in an expression),
69 and thus need an explicit termination if used as the last item in a statement.
71 Any simple statement may optionally be followed by a I<SINGLE> modifier,
72 just before the terminating semicolon (or block ending). The possible
80 The C<if> and C<unless> modifiers have the expected semantics,
81 presuming you're a speaker of English. The C<while> and C<until>
82 modifiers also have the usual "while loop" semantics (conditional
83 evaluated first), except when applied to a do-BLOCK (or to the
84 now-deprecated do-SUBROUTINE statement), in which case the block
85 executes once before the conditional is evaluated. This is so that you
91 } until $line eq ".\n";
93 See L<perlfunc/do>. Note also that the loop control
94 statements described later will I<NOT> work in this construct, since
95 modifiers don't take loop labels. Sorry. You can always wrap
96 another block around it to do that sort of thing.
98 =head2 Compound statements
100 In Perl, a sequence of statements that defines a scope is called a block.
101 Sometimes a block is delimited by the file containing it (in the case
102 of a required file, or the program as a whole), and sometimes a block
103 is delimited by the extent of a string (in the case of an eval).
105 But generally, a block is delimited by curly brackets, also known as braces.
106 We will call this syntactic construct a BLOCK.
108 The following compound statements may be used to control flow:
111 if (EXPR) BLOCK else BLOCK
112 if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
113 LABEL while (EXPR) BLOCK
114 LABEL while (EXPR) BLOCK continue BLOCK
115 LABEL for (EXPR; EXPR; EXPR) BLOCK
116 LABEL foreach VAR (LIST) BLOCK
117 LABEL BLOCK continue BLOCK
119 Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
120 not statements. This means that the curly brackets are I<required>--no
121 dangling statements allowed. If you want to write conditionals without
122 curly brackets there are several other ways to do it. The following
123 all do the same thing:
125 if (!open(FOO)) { die "Can't open $FOO: $!"; }
126 die "Can't open $FOO: $!" unless open(FOO);
127 open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
128 open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
129 # a bit exotic, that last one
131 The C<if> statement is straightforward. Since BLOCKs are always
132 bounded by curly brackets, there is never any ambiguity about which
133 C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
134 the sense of the test is reversed.
136 The C<while> statement executes the block as long as the expression is
137 true (does not evaluate to the null string or 0 or "0"). The LABEL is
138 optional, and if present, consists of an identifier followed by a colon.
139 The LABEL identifies the loop for the loop control statements C<next>,
140 C<last>, and C<redo>. If the LABEL is omitted, the loop control statement
141 refers to the innermost enclosing loop. This may include dynamically
142 looking back your call-stack at run time to find the LABEL. Such
143 desperate behavior triggers a warning if you use the B<-w> flag.
145 If there is a C<continue> BLOCK, it is always executed just before the
146 conditional is about to be evaluated again, just like the third part of a
147 C<for> loop in C. Thus it can be used to increment a loop variable, even
148 when the loop has been continued via the C<next> statement (which is
149 similar to the C C<continue> statement).
153 The C<next> command is like the C<continue> statement in C; it starts
154 the next iteration of the loop:
156 LINE: while (<STDIN>) {
157 next LINE if /^#/; # discard comments
161 The C<last> command is like the C<break> statement in C (as used in
162 loops); it immediately exits the loop in question. The
163 C<continue> block, if any, is not executed:
165 LINE: while (<STDIN>) {
166 last LINE if /^$/; # exit when done with header
170 The C<redo> command restarts the loop block without evaluating the
171 conditional again. The C<continue> block, if any, is I<not> executed.
172 This command is normally used by programs that want to lie to themselves
173 about what was just input.
175 For example, when processing a file like F</etc/termcap>.
176 If your input lines might end in backslashes to indicate continuation, you
177 want to skip ahead and get the next record.
188 which is Perl short-hand for the more explicitly written version:
190 LINE: while ($line = <ARGV>) {
192 if ($line =~ s/\\$//) {
194 redo LINE unless eof(); # not eof(ARGV)!
199 Or here's a a simpleminded Pascal comment stripper (warning: assumes no { or } in strings)
201 LINE: while (<STDIN>) {
202 while (s|({.*}.*){.*}|$1 |) {}
207 if (/}/) { # end of comment?
216 Note that if there were a C<continue> block on the above code, it would get
217 executed even on discarded lines.
219 If the word C<while> is replaced by the word C<until>, the sense of the
220 test is reversed, but the conditional is still tested before the first
223 In either the C<if> or the C<while> statement, you may replace "(EXPR)"
224 with a BLOCK, and the conditional is true if the value of the last
225 statement in that block is true. While this "feature" continues to work in
226 version 5, it has been deprecated, so please change any occurrences of "if BLOCK" to
231 Perl's C-style C<for> loop works exactly like the corresponding C<while> loop;
232 that means that this:
234 for ($i = 1; $i < 10; $i++) {
247 Besides the normal array index looping, C<for> can lend itself
248 to many other interesting applications. Here's one that avoids the
249 problem you get into if you explicitly test for end-of-file on
250 an interactive file descriptor causing your program to appear to
253 $on_a_tty = -t STDIN && -t STDOUT;
254 sub prompt { print "yes? " if $on_a_tty }
255 for ( prompt(); <STDIN>; prompt() ) {
261 The C<foreach> loop iterates over a normal list value and sets the
262 variable VAR to be each element of the list in turn. The variable is
263 implicitly local to the loop and regains its former value upon exiting the
264 loop. If the variable was previously declared with C<my>, it uses that
265 variable instead of the global one, but it's still localized to the loop.
266 This can cause problems if you have subroutine or format declarations
267 within that block's scope.
269 The C<foreach> keyword is actually a synonym for the C<for> keyword, so
270 you can use C<foreach> for readability or C<for> for brevity. If VAR is
271 omitted, $_ is set to each value. If LIST is an actual array (as opposed
272 to an expression returning a list value), you can modify each element of
273 the array by modifying VAR inside the loop. That's because the C<foreach>
274 loop index variable is an implicit alias for each item in the list that
279 for (@ary) { s/foo/bar/ }
281 foreach $elem (@elements) {
285 for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
286 print $count, "\n"; sleep(1);
289 for (1..15) { print "Merry Christmas\n"; }
291 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
292 print "Item: $item\n";
295 Here's how a C programmer might code up a particular algorithm in Perl:
297 for ($i = 0; $i < @ary1; $i++) {
298 for ($j = 0; $j < @ary2; $j++) {
299 if ($ary1[$i] > $ary2[$j]) {
300 last; # can't go to outer :-(
302 $ary1[$i] += $ary2[$j];
304 # this is where that last takes me
307 Whereas here's how a Perl programmer more confortable with the idiom might
310 OUTER: foreach $wid (@ary1) {
311 INNER: foreach $jet (@ary2) {
312 next OUTER if $wid > $jet;
317 See how much easier this is? It's cleaner, safer, and faster. It's
318 cleaner because it's less noisy. It's safer because if code gets added
319 between the inner and outer loops later on, the new code won't be
320 accidentally executed, the C<next> explicitly iterates the other loop
321 rather than merely terminating the inner one. And it's faster because
322 Perl executes a C<foreach> statement more rapidly than it would the
323 equivalent C<for> loop.
325 =head2 Basic BLOCKs and Switch Statements
327 A BLOCK by itself (labeled or not) is semantically equivalent to a loop
328 that executes once. Thus you can use any of the loop control
329 statements in it to leave or restart the block. (Note that this
330 is I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief C<do{}> blocks,
331 which do I<NOT> count as loops.) The C<continue> block
334 The BLOCK construct is particularly nice for doing case
338 if (/^abc/) { $abc = 1; last SWITCH; }
339 if (/^def/) { $def = 1; last SWITCH; }
340 if (/^xyz/) { $xyz = 1; last SWITCH; }
344 There is no official switch statement in Perl, because there are
345 already several ways to write the equivalent. In addition to the
346 above, you could write
349 $abc = 1, last SWITCH if /^abc/;
350 $def = 1, last SWITCH if /^def/;
351 $xyz = 1, last SWITCH if /^xyz/;
355 (That's actually not as strange as it looks once you realize that you can
356 use loop control "operators" within an expression, That's just the normal
362 /^abc/ && do { $abc = 1; last SWITCH; };
363 /^def/ && do { $def = 1; last SWITCH; };
364 /^xyz/ && do { $xyz = 1; last SWITCH; };
368 or formatted so it stands out more as a "proper" switch statement:
391 /^abc/ and $abc = 1, last SWITCH;
392 /^def/ and $def = 1, last SWITCH;
393 /^xyz/ and $xyz = 1, last SWITCH;
409 A common idiom for a switch statement is to use C<foreach>'s aliasing to make
410 a temporary assignment to $_ for convenient matching:
412 SWITCH: for ($where) {
413 /In Card Names/ && do { push @flags, '-e'; last; };
414 /Anywhere/ && do { push @flags, '-h'; last; };
415 /In Rulings/ && do { last; };
416 die "unknown value for form variable where: `$where'";
419 Another interesting approach to a switch statement is arrange
420 for a C<do> block to return the proper value:
423 if ($flag & O_RDONLY) { "r" }
424 elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
425 elsif ($flag & O_RDWR) {
426 if ($flag & O_CREAT) { "w+" }
427 else { ($flag & O_APPEND) ? "a+" : "r+" }
433 Although not for the faint of heart, Perl does support a C<goto> statement.
434 A loop's LABEL is not actually a valid target for a C<goto>;
435 it's just the name of the loop. There are three forms: goto-LABEL,
436 goto-EXPR, and goto-&NAME.
438 The goto-LABEL form finds the statement labeled with LABEL and resumes
439 execution there. It may not be used to go into any construct that
440 requires initialization, such as a subroutine or a foreach loop. It
441 also can't be used to go into a construct that is optimized away. It
442 can be used to go almost anywhere else within the dynamic scope,
443 including out of subroutines, but it's usually better to use some other
444 construct such as last or die. The author of Perl has never felt the
445 need to use this form of goto (in Perl, that is--C is another matter).
447 The goto-EXPR form expects a label name, whose scope will be resolved
448 dynamically. This allows for computed gotos per FORTRAN, but isn't
449 necessarily recommended if you're optimizing for maintainability:
451 goto ("FOO", "BAR", "GLARCH")[$i];
453 The goto-&NAME form is highly magical, and substitutes a call to the
454 named subroutine for the currently running subroutine. This is used by
455 AUTOLOAD() subroutines that wish to load another subroutine and then
456 pretend that the other subroutine had been called in the first place
457 (except that any modifications to @_ in the current subroutine are
458 propagated to the other subroutine.) After the C<goto>, not even caller()
459 will be able to tell that this routine was called first.
461 In almost all cases like this, it's usually a far, far better idea to use the
462 structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
463 resorting to a C<goto>. For certain applications, the catch and throw pair of
464 C<eval{}> and die() for exception processing can also be a prudent approach.
466 =head2 PODs: Embedded Documentation
468 Perl has a mechanism for intermixing documentation with source code.
469 While it's expecting the beginning of a new statement, if the compiler
470 encounters a line that begins with an equal sign and a word, like this
472 =head1 Here There Be Pods!
474 Then that text and all remaining text up through and including a line
475 beginning with C<=cut> will be ignored. The format of the intervening
476 text is described in L<perlpod>.
478 This allows you to intermix your source code
479 and your documentation text freely, as in
483 The snazzle() function will behave in the most spectacular
484 form that you can possibly imagine, not even excepting
485 cybernetic pyrotechnics.
487 =cut back to the compiler, nuff of this pod stuff!
494 Note that pod translators should only look at paragraphs beginning
495 with a pod diretive (it makes parsing easier), whereas the compiler
496 actually knows to look for pod escapes even in the middle of a
497 paragraph. This means that the following secret stuff will be
498 ignored by both the compiler and the translators.
502 warn "Neither POD nor CODE!?"
506 You probably shouldn't rely upon the warn() being podded out forever.
507 Not all pod translators are well-behaved in this regard, and perhaps
508 the compiler will become pickier.