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 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, because 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, because
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: $!";
129 # a bit exotic, that last one
131 The C<if> statement is straightforward. Because BLOCKs are always
132 bounded by curly brackets, there is never any ambiguity about which
133 C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
134 the sense of the test is reversed.
136 The C<while> statement executes the block as long as the expression is
137 true (does not evaluate to the null string or 0 or "0"). The LABEL is
138 optional, and if present, consists of an identifier followed by a colon.
139 The LABEL identifies the loop for the loop control statements C<next>,
140 C<last>, and C<redo>. If the LABEL is omitted, the loop control statement
141 refers to the innermost enclosing loop. This may include dynamically
142 looking back your call-stack at run time to find the LABEL. Such
143 desperate behavior triggers a warning if you use the B<-w> flag.
145 If there is a C<continue> BLOCK, it is always executed just before the
146 conditional is about to be evaluated again, just like the third part of a
147 C<for> loop in C. Thus it can be used to increment a loop variable, even
148 when the loop has been continued via the C<next> statement (which is
149 similar to the C C<continue> statement).
153 The C<next> command is like the C<continue> statement in C; it starts
154 the next iteration of the loop:
156 LINE: while (<STDIN>) {
157 next LINE if /^#/; # discard comments
161 The C<last> command is like the C<break> statement in C (as used in
162 loops); it immediately exits the loop in question. The
163 C<continue> block, if any, is not executed:
165 LINE: while (<STDIN>) {
166 last LINE if /^$/; # 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 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 (There is one minor difference: The first form implies a lexical scope
248 for variables declared with C<my> in the initialization expression.)
250 Besides the normal array index looping, C<for> can lend itself
251 to many other interesting applications. Here's one that avoids the
252 problem you get into if you explicitly test for end-of-file on
253 an interactive file descriptor causing your program to appear to
256 $on_a_tty = -t STDIN && -t STDOUT;
257 sub prompt { print "yes? " if $on_a_tty }
258 for ( prompt(); <STDIN>; prompt() ) {
264 The C<foreach> loop iterates over a normal list value and sets the
265 variable VAR to be each element of the list in turn. If the variable
266 is preceded with the keyword C<my>, then it is lexically scoped, and
267 is therefore visible only within the loop. Otherwise, the variable is
268 implicitly local to the loop and regains its former value upon exiting
269 the loop. If the variable was previously declared with C<my>, it uses
270 that variable instead of the global one, but it's still localized to
271 the loop. (Note that a lexically scoped variable can cause problems
272 with you have subroutine or format declarations.)
274 The C<foreach> keyword is actually a synonym for the C<for> keyword, so
275 you can use C<foreach> for readability or C<for> for brevity. If VAR is
276 omitted, $_ is set to each value. If LIST is an actual array (as opposed
277 to an expression returning a list value), you can modify each element of
278 the array by modifying VAR inside the loop. That's because the C<foreach>
279 loop index variable is an implicit alias for each item in the list that
284 for (@ary) { s/foo/bar/ }
286 foreach my $elem (@elements) {
290 for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
291 print $count, "\n"; sleep(1);
294 for (1..15) { print "Merry Christmas\n"; }
296 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
297 print "Item: $item\n";
300 Here's how a C programmer might code up a particular algorithm in Perl:
302 for (my $i = 0; $i < @ary1; $i++) {
303 for (my $j = 0; $j < @ary2; $j++) {
304 if ($ary1[$i] > $ary2[$j]) {
305 last; # can't go to outer :-(
307 $ary1[$i] += $ary2[$j];
309 # this is where that last takes me
312 Whereas here's how a Perl programmer more comfortable with the idiom might
315 OUTER: foreach my $wid (@ary1) {
316 INNER: foreach my $jet (@ary2) {
317 next OUTER if $wid > $jet;
322 See how much easier this is? It's cleaner, safer, and faster. It's
323 cleaner because it's less noisy. It's safer because if code gets added
324 between the inner and outer loops later on, the new code won't be
325 accidentally executed. The C<next> explicitly iterates the other loop
326 rather than merely terminating the inner one. And it's faster because
327 Perl executes a C<foreach> statement more rapidly than it would the
328 equivalent C<for> loop.
330 =head2 Basic BLOCKs and Switch Statements
332 A BLOCK by itself (labeled or not) is semantically equivalent to a
333 loop that executes once. Thus you can use any of the loop control
334 statements in it to leave or restart the block. (Note that this is
335 I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
336 C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
339 The BLOCK construct is particularly nice for doing case
343 if (/^abc/) { $abc = 1; last SWITCH; }
344 if (/^def/) { $def = 1; last SWITCH; }
345 if (/^xyz/) { $xyz = 1; last SWITCH; }
349 There is no official switch statement in Perl, because there are
350 already several ways to write the equivalent. In addition to the
351 above, you could write
354 $abc = 1, last SWITCH if /^abc/;
355 $def = 1, last SWITCH if /^def/;
356 $xyz = 1, last SWITCH if /^xyz/;
360 (That's actually not as strange as it looks once you realize that you can
361 use loop control "operators" within an expression, That's just the normal
367 /^abc/ && do { $abc = 1; last SWITCH; };
368 /^def/ && do { $def = 1; last SWITCH; };
369 /^xyz/ && do { $xyz = 1; last SWITCH; };
373 or formatted so it stands out more as a "proper" switch statement:
396 /^abc/ and $abc = 1, last SWITCH;
397 /^def/ and $def = 1, last SWITCH;
398 /^xyz/ and $xyz = 1, last SWITCH;
414 A common idiom for a switch statement is to use C<foreach>'s aliasing to make
415 a temporary assignment to $_ for convenient matching:
417 SWITCH: for ($where) {
418 /In Card Names/ && do { push @flags, '-e'; last; };
419 /Anywhere/ && do { push @flags, '-h'; last; };
420 /In Rulings/ && do { last; };
421 die "unknown value for form variable where: `$where'";
424 Another interesting approach to a switch statement is arrange
425 for a C<do> block to return the proper value:
428 if ($flag & O_RDONLY) { "r" }
429 elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
430 elsif ($flag & O_RDWR) {
431 if ($flag & O_CREAT) { "w+" }
432 else { ($flag & O_APPEND) ? "a+" : "r+" }
438 Although not for the faint of heart, Perl does support a C<goto> statement.
439 A loop's LABEL is not actually a valid target for a C<goto>;
440 it's just the name of the loop. There are three forms: goto-LABEL,
441 goto-EXPR, and goto-&NAME.
443 The goto-LABEL form finds the statement labeled with LABEL and resumes
444 execution there. It may not be used to go into any construct that
445 requires initialization, such as a subroutine or a foreach loop. It
446 also can't be used to go into a construct that is optimized away. It
447 can be used to go almost anywhere else within the dynamic scope,
448 including out of subroutines, but it's usually better to use some other
449 construct such as last or die. The author of Perl has never felt the
450 need to use this form of goto (in Perl, that is--C is another matter).
452 The goto-EXPR form expects a label name, whose scope will be resolved
453 dynamically. This allows for computed gotos per FORTRAN, but isn't
454 necessarily recommended if you're optimizing for maintainability:
456 goto ("FOO", "BAR", "GLARCH")[$i];
458 The goto-&NAME form is highly magical, and substitutes a call to the
459 named subroutine for the currently running subroutine. This is used by
460 AUTOLOAD() subroutines that wish to load another subroutine and then
461 pretend that the other subroutine had been called in the first place
462 (except that any modifications to @_ in the current subroutine are
463 propagated to the other subroutine.) After the C<goto>, not even caller()
464 will be able to tell that this routine was called first.
466 In almost all cases like this, it's usually a far, far better idea to use the
467 structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
468 resorting to a C<goto>. For certain applications, the catch and throw pair of
469 C<eval{}> and die() for exception processing can also be a prudent approach.
471 =head2 PODs: Embedded Documentation
473 Perl has a mechanism for intermixing documentation with source code.
474 While it's expecting the beginning of a new statement, if the compiler
475 encounters a line that begins with an equal sign and a word, like this
477 =head1 Here There Be Pods!
479 Then that text and all remaining text up through and including a line
480 beginning with C<=cut> will be ignored. The format of the intervening
481 text is described in L<perlpod>.
483 This allows you to intermix your source code
484 and your documentation text freely, as in
488 The snazzle() function will behave in the most spectacular
489 form that you can possibly imagine, not even excepting
490 cybernetic pyrotechnics.
492 =cut back to the compiler, nuff of this pod stuff!
499 Note that pod translators should look at only paragraphs beginning
500 with a pod directive (it makes parsing easier), whereas the compiler
501 actually knows to look for pod escapes even in the middle of a
502 paragraph. This means that the following secret stuff will be
503 ignored by both the compiler and the translators.
507 warn "Neither POD nor CODE!?"
511 You probably shouldn't rely upon the warn() being podded out forever.
512 Not all pod translators are well-behaved in this regard, and perhaps
513 the compiler will become pickier.