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 without defining it by saying C<sub name>, thus:
45 $me = myname $0 or die "can't get myname";
47 Note that it functions as a list operator, not as a unary operator; so
48 be careful to use C<or> instead of C<||> in this case. However, if
49 you were to declare the subroutine as C<sub myname ($)>, then
50 C<myname> would functonion as a unary operator, so either C<or> or
53 Subroutines declarations can also be loaded up with the C<require> statement
54 or both loaded and imported into your namespace with a C<use> statement.
55 See L<perlmod> for details on this.
57 A statement sequence may contain declarations of lexically-scoped
58 variables, but apart from declaring a variable name, the declaration acts
59 like an ordinary statement, and is elaborated within the sequence of
60 statements as if it were an ordinary statement. That means it actually
61 has both compile-time and run-time effects.
63 =head2 Simple statements
65 The only kind of simple statement is an expression evaluated for its
66 side effects. Every simple statement must be terminated with a
67 semicolon, unless it is the final statement in a block, in which case
68 the semicolon is optional. (A semicolon is still encouraged there if the
69 block takes up more than one line, because you may eventually add another line.)
70 Note that there are some operators like C<eval {}> and C<do {}> that look
71 like compound statements, but aren't (they're just TERMs in an expression),
72 and thus need an explicit termination if used as the last item in a statement.
74 Any simple statement may optionally be followed by a I<SINGLE> modifier,
75 just before the terminating semicolon (or block ending). The possible
84 The C<if> and C<unless> modifiers have the expected semantics,
85 presuming you're a speaker of English. The C<foreach> modifier is an
86 iterator: For each value in EXPR, it aliases $_ to the value and
87 executes the statement. The C<while> and C<until> modifiers have the
88 usual "while loop" semantics (conditional evaluated first), except
89 when applied to a do-BLOCK (or to the now-deprecated do-SUBROUTINE
90 statement), in which case the block executes once before the
91 conditional is evaluated. This is so that you can write loops like:
96 } until $line eq ".\n";
98 See L<perlfunc/do>. Note also that the loop control
99 statements described later will I<NOT> work in this construct, because
100 modifiers don't take loop labels. Sorry. You can always wrap
101 another block around it to do that sort of thing.
103 =head2 Compound statements
105 In Perl, a sequence of statements that defines a scope is called a block.
106 Sometimes a block is delimited by the file containing it (in the case
107 of a required file, or the program as a whole), and sometimes a block
108 is delimited by the extent of a string (in the case of an eval).
110 But generally, a block is delimited by curly brackets, also known as braces.
111 We will call this syntactic construct a BLOCK.
113 The following compound statements may be used to control flow:
116 if (EXPR) BLOCK else BLOCK
117 if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
118 LABEL while (EXPR) BLOCK
119 LABEL while (EXPR) BLOCK continue BLOCK
120 LABEL for (EXPR; EXPR; EXPR) BLOCK
121 LABEL foreach VAR (LIST) BLOCK
122 LABEL BLOCK continue BLOCK
124 Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
125 not statements. This means that the curly brackets are I<required>--no
126 dangling statements allowed. If you want to write conditionals without
127 curly brackets there are several other ways to do it. The following
128 all do the same thing:
130 if (!open(FOO)) { die "Can't open $FOO: $!"; }
131 die "Can't open $FOO: $!" unless open(FOO);
132 open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
133 open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
134 # a bit exotic, that last one
136 The C<if> statement is straightforward. Because BLOCKs are always
137 bounded by curly brackets, there is never any ambiguity about which
138 C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
139 the sense of the test is reversed.
141 The C<while> statement executes the block as long as the expression is
142 true (does not evaluate to the null string or 0 or "0"). The LABEL is
143 optional, and if present, consists of an identifier followed by a colon.
144 The LABEL identifies the loop for the loop control statements C<next>,
145 C<last>, and C<redo>. If the LABEL is omitted, the loop control statement
146 refers to the innermost enclosing loop. This may include dynamically
147 looking back your call-stack at run time to find the LABEL. Such
148 desperate behavior triggers a warning if you use the B<-w> flag.
150 If there is a C<continue> BLOCK, it is always executed just before the
151 conditional is about to be evaluated again, just like the third part of a
152 C<for> loop in C. Thus it can be used to increment a loop variable, even
153 when the loop has been continued via the C<next> statement (which is
154 similar to the C C<continue> statement).
158 The C<next> command is like the C<continue> statement in C; it starts
159 the next iteration of the loop:
161 LINE: while (<STDIN>) {
162 next LINE if /^#/; # discard comments
166 The C<last> command is like the C<break> statement in C (as used in
167 loops); it immediately exits the loop in question. The
168 C<continue> block, if any, is not executed:
170 LINE: while (<STDIN>) {
171 last LINE if /^$/; # exit when done with header
175 The C<redo> command restarts the loop block without evaluating the
176 conditional again. The C<continue> block, if any, is I<not> executed.
177 This command is normally used by programs that want to lie to themselves
178 about what was just input.
180 For example, when processing a file like F</etc/termcap>.
181 If your input lines might end in backslashes to indicate continuation, you
182 want to skip ahead and get the next record.
193 which is Perl short-hand for the more explicitly written version:
195 LINE: while (defined($line = <ARGV>)) {
197 if ($line =~ s/\\$//) {
199 redo LINE unless eof(); # not eof(ARGV)!
204 Or here's a simpleminded Pascal comment stripper (warning: assumes no
207 LINE: while (<STDIN>) {
208 while (s|({.*}.*){.*}|$1 |) {}
213 if (/}/) { # end of comment?
222 Note that if there were a C<continue> block on the above code, it would get
223 executed even on discarded lines.
225 If the word C<while> is replaced by the word C<until>, the sense of the
226 test is reversed, but the conditional is still tested before the first
229 The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
230 available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
234 Perl's C-style C<for> loop works exactly like the corresponding C<while> loop;
235 that means that this:
237 for ($i = 1; $i < 10; $i++) {
250 (There is one minor difference: The first form implies a lexical scope
251 for variables declared with C<my> in the initialization expression.)
253 Besides the normal array index looping, C<for> can lend itself
254 to many other interesting applications. Here's one that avoids the
255 problem you get into if you explicitly test for end-of-file on
256 an interactive file descriptor causing your program to appear to
259 $on_a_tty = -t STDIN && -t STDOUT;
260 sub prompt { print "yes? " if $on_a_tty }
261 for ( prompt(); <STDIN>; prompt() ) {
267 The C<foreach> loop iterates over a normal list value and sets the
268 variable VAR to be each element of the list in turn. If the variable
269 is preceded with the keyword C<my>, then it is lexically scoped, and
270 is therefore visible only within the loop. Otherwise, the variable is
271 implicitly local to the loop and regains its former value upon exiting
272 the loop. If the variable was previously declared with C<my>, it uses
273 that variable instead of the global one, but it's still localized to
274 the loop. (Note that a lexically scoped variable can cause problems
275 if you have subroutine or format declarations within the loop which
278 The C<foreach> keyword is actually a synonym for the C<for> keyword, so
279 you can use C<foreach> for readability or C<for> for brevity. If VAR is
280 omitted, $_ is set to each value. If any element of LIST is an lvalue,
281 you can modify it by modifying VAR inside the loop. That's because
282 the C<foreach> loop index variable is an implicit alias for each item
283 in the list that you're looping over.
285 If any part of LIST is an array, C<foreach> will get very confused if
286 you add or remove elements within the loop body, for example with
287 C<splice>. So don't do that.
289 C<foreach> probably won't do what you expect if VAR is a tied or other
290 special variable. Don't do that either.
294 for (@ary) { s/foo/bar/ }
296 foreach my $elem (@elements) {
300 for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
301 print $count, "\n"; sleep(1);
304 for (1..15) { print "Merry Christmas\n"; }
306 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
307 print "Item: $item\n";
310 Here's how a C programmer might code up a particular algorithm in Perl:
312 for (my $i = 0; $i < @ary1; $i++) {
313 for (my $j = 0; $j < @ary2; $j++) {
314 if ($ary1[$i] > $ary2[$j]) {
315 last; # can't go to outer :-(
317 $ary1[$i] += $ary2[$j];
319 # this is where that last takes me
322 Whereas here's how a Perl programmer more comfortable with the idiom might
325 OUTER: foreach my $wid (@ary1) {
326 INNER: foreach my $jet (@ary2) {
327 next OUTER if $wid > $jet;
332 See how much easier this is? It's cleaner, safer, and faster. It's
333 cleaner because it's less noisy. It's safer because if code gets added
334 between the inner and outer loops later on, the new code won't be
335 accidentally executed. The C<next> explicitly iterates the other loop
336 rather than merely terminating the inner one. And it's faster because
337 Perl executes a C<foreach> statement more rapidly than it would the
338 equivalent C<for> loop.
340 =head2 Basic BLOCKs and Switch Statements
342 A BLOCK by itself (labeled or not) is semantically equivalent to a
343 loop that executes once. Thus you can use any of the loop control
344 statements in it to leave or restart the block. (Note that this is
345 I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
346 C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
349 The BLOCK construct is particularly nice for doing case
353 if (/^abc/) { $abc = 1; last SWITCH; }
354 if (/^def/) { $def = 1; last SWITCH; }
355 if (/^xyz/) { $xyz = 1; last SWITCH; }
359 There is no official switch statement in Perl, because there are
360 already several ways to write the equivalent. In addition to the
361 above, you could write
364 $abc = 1, last SWITCH if /^abc/;
365 $def = 1, last SWITCH if /^def/;
366 $xyz = 1, last SWITCH if /^xyz/;
370 (That's actually not as strange as it looks once you realize that you can
371 use loop control "operators" within an expression, That's just the normal
377 /^abc/ && do { $abc = 1; last SWITCH; };
378 /^def/ && do { $def = 1; last SWITCH; };
379 /^xyz/ && do { $xyz = 1; last SWITCH; };
383 or formatted so it stands out more as a "proper" switch statement:
406 /^abc/ and $abc = 1, last SWITCH;
407 /^def/ and $def = 1, last SWITCH;
408 /^xyz/ and $xyz = 1, last SWITCH;
424 A common idiom for a switch statement is to use C<foreach>'s aliasing to make
425 a temporary assignment to $_ for convenient matching:
427 SWITCH: for ($where) {
428 /In Card Names/ && do { push @flags, '-e'; last; };
429 /Anywhere/ && do { push @flags, '-h'; last; };
430 /In Rulings/ && do { last; };
431 die "unknown value for form variable where: `$where'";
434 Another interesting approach to a switch statement is arrange
435 for a C<do> block to return the proper value:
438 if ($flag & O_RDONLY) { "r" }
439 elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
440 elsif ($flag & O_RDWR) {
441 if ($flag & O_CREAT) { "w+" }
442 else { ($flag & O_APPEND) ? "a+" : "r+" }
448 Although not for the faint of heart, Perl does support a C<goto> statement.
449 A loop's LABEL is not actually a valid target for a C<goto>;
450 it's just the name of the loop. There are three forms: goto-LABEL,
451 goto-EXPR, and goto-&NAME.
453 The goto-LABEL form finds the statement labeled with LABEL and resumes
454 execution there. It may not be used to go into any construct that
455 requires initialization, such as a subroutine or a foreach loop. It
456 also can't be used to go into a construct that is optimized away. It
457 can be used to go almost anywhere else within the dynamic scope,
458 including out of subroutines, but it's usually better to use some other
459 construct such as last or die. The author of Perl has never felt the
460 need to use this form of goto (in Perl, that is--C is another matter).
462 The goto-EXPR form expects a label name, whose scope will be resolved
463 dynamically. This allows for computed gotos per FORTRAN, but isn't
464 necessarily recommended if you're optimizing for maintainability:
466 goto ("FOO", "BAR", "GLARCH")[$i];
468 The goto-&NAME form is highly magical, and substitutes a call to the
469 named subroutine for the currently running subroutine. This is used by
470 AUTOLOAD() subroutines that wish to load another subroutine and then
471 pretend that the other subroutine had been called in the first place
472 (except that any modifications to @_ in the current subroutine are
473 propagated to the other subroutine.) After the C<goto>, not even caller()
474 will be able to tell that this routine was called first.
476 In almost all cases like this, it's usually a far, far better idea to use the
477 structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
478 resorting to a C<goto>. For certain applications, the catch and throw pair of
479 C<eval{}> and die() for exception processing can also be a prudent approach.
481 =head2 PODs: Embedded Documentation
483 Perl has a mechanism for intermixing documentation with source code.
484 While it's expecting the beginning of a new statement, if the compiler
485 encounters a line that begins with an equal sign and a word, like this
487 =head1 Here There Be Pods!
489 Then that text and all remaining text up through and including a line
490 beginning with C<=cut> will be ignored. The format of the intervening
491 text is described in L<perlpod>.
493 This allows you to intermix your source code
494 and your documentation text freely, as in
498 The snazzle() function will behave in the most spectacular
499 form that you can possibly imagine, not even excepting
500 cybernetic pyrotechnics.
502 =cut back to the compiler, nuff of this pod stuff!
509 Note that pod translators should look at only paragraphs beginning
510 with a pod directive (it makes parsing easier), whereas the compiler
511 actually knows to look for pod escapes even in the middle of a
512 paragraph. This means that the following secret stuff will be
513 ignored by both the compiler and the translators.
517 warn "Neither POD nor CODE!?"
521 You probably shouldn't rely upon the warn() being podded out forever.
522 Not all pod translators are well-behaved in this regard, and perhaps
523 the compiler will become pickier.
525 One may also use pod directives to quickly comment out a section
528 =head2 Plain Old Comments (Not!)
530 Much like the C preprocessor, perl can process line directives. Using
531 this, one can control perl's idea of filenames and line numbers in
532 error or warning messages (especially for strings that are processed
533 with eval()). The syntax for this mechanism is the same as for most
534 C preprocessors: it matches the regular expression
535 C</^#\s*line\s+(\d+)\s*(?:\s"([^"]*)")?/> with C<$1> being the line
536 number for the next line, and C<$2> being the optional filename
537 (specified within quotes).
539 Here are some examples that you should be able to type into your command
544 # the `#' on the previous line must be the first char on line
547 foo at bzzzt line 201.
551 eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
556 eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
558 foo at foo bar line 200.
562 eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
565 foo at goop line 345.