Perl is, for the most part, a free-form language. (The only exception to this is format declarations, for obvious reasons.) Comments are indicated by the "#" character, and extend to the end of the line. If you attempt to use /* */ C-style comments, it will be interpreted either as division or pattern matching, depending on the context, and C++ // comments just look like a null regular expression, So don't do that.
A declaration can be put anywhere a statement can, but has no effect on the execution of the primary sequence of statements--declarations all take effect at compile time. Typically all the declarations are put at the beginning or the end of the script.
As of Perl 5, declaring a subroutine allows a subroutine name to be used as if it were a list operator from that point forward in the program. You can declare a subroutine without defining it by saying just
sub myname;
$me = myname $0 or die "can't get myname";
Note that it functions as a list operator though, not a unary
operator, so be careful to use or instead of || there.
Subroutines declarations can also be imported by a use statement.
Also as of Perl 5, a statement sequence may contain declarations of lexically scoped variables, but apart from declaring a variable name, the declaration acts like an ordinary statement, and is elaborated within the sequence of statements as if it were an ordinary statement.
Any simple statement may optionally be followed by a SINGL&/I; modifier, just before the terminating semicolon (or block ending). The possible modifiers are:
if EXPR
unless EXPR
while EXPR
until EXPR
The if and unless modifiers have the expected semantics,
presuming you're a speaker of English. The while and until
modifiers also have the usual "while loop" semantics (conditional
evaluated first), except when applied to a do-BLOCK (or to the
now-deprecated do-SUBROUTINE statement), in which case the block
executes once before the conditional is evaluated. This is so that you
can write loops like:
do {
$_ = <STDIN>;
...
} until $_ eq ".\n";
See perlfunc/do. Note also that the loop control
statements described later will NOT work in this construct, since
modifiers don't take loop labels. Sorry. You can always wrap
another block around it to do that sort of thing.)
But generally, a block is delimited by curly brackets, also known as braces. We will call this syntactic construct a BLOCK.
The following compound statements may be used to control flow:
if (EXPR) BLOCK
if (EXPR) BLOCK else BLOCK
if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
LABEL while (EXPR) BLOCK
LABEL while (EXPR) BLOCK continue BLOCK
LABEL for (EXPR; EXPR; EXPR) BLOCK
LABEL foreach VAR (ARRAY) BLOCK
LABEL BLOCK continue BLOCK
Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
not statements. This means that the curly brackets are required--no
dangling statements allowed. If you want to write conditionals without
curly brackets there are several other ways to do it. The following
all do the same thing:
if (!open(FOO)) { die "Can't open $FOO: $!"; }
die "Can't open $FOO: $!" unless open(FOO);
open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
# a bit exotic, that last one
The if statement is straightforward. Since BLOCKs are always
bounded by curly brackets, there is never any ambiguity about which
if an else goes with. If you use unless in place of if,
the sense of the test is reversed.
The while statement executes the block as long as the expression is true (does not evaluate to the null string or 0 or "0"). The LABEL is optional, and if present, consists of an identifier followed by a colon. The LABEL identifies the loop for the loop control statements next , last , and redo (see below). If there is a continue BLOCK, it is always executed just before the conditional is about to be evaluated again, just like the third part of a for loop in C. Thus it can be used to increment a loop variable, even when the loop has been continued via the next statement (which is similar to the C continue statement).
If the word while is replaced by the word until, the sense of the test is reversed, but the conditional is still tested before the first iteration.
In either the if or the while statement, you may replace "(EXPR)" with a BLOCK, and the conditional is true if the value of the last statement in that block is true. (This feature continues to work in Perl 5 but is deprecated. Please change any occurrences of "if BLOCK" to "if (do BLOCK)".)
The C-style for loop works exactly like the corresponding while loop:
for ($i = 1; $i < 10; $i++) {
...
}
is the same as
$i = 1;
while ($i < 10) {
...
} continue {
$i++;
}
The foreach loop iterates over a normal list value and sets the
variable VAR to be each element of the list in turn. The variable is
implicitly local to the loop (unless declared previously with
my
),
and regains its former value upon exiting the loop. The foreach
keyword is actually a synonym for the for keyword, so you can use
foreach for readability or for for brevity. If VAR is omitted, $_
is set to each value. If ARRAY is an actual array (as opposed to an
expression returning a list value), you can modify each element of the
array by modifying VAR inside the loop. Examples:
for (@ary) { s/foo/bar/; }
foreach $elem (@elements) {
$elem *= 2;
}
for ((10,9,8,7,6,5,4,3,2,1,'BOOM')) {
print $_, "\n"; sleep(1);
}
for (1..15) { print "Merry Christmas\n"; }
foreach $item (split(/:[\\\n:]*/, $ENV{'TERMCAP'})) {
print "Item: $item\n";
}
A BLOCK by itself (labeled or not) is semantically equivalent to a loop
that executes once. Thus you can use any of the loop control
statements in it to leave or restart the block. The continue block
is optional. This construct is particularly nice for doing case
structures.
SWITCH: {
if (/^abc/) { $abc = 1; last SWITCH; }
if (/^def/) { $def = 1; last SWITCH; }
if (/^xyz/) { $xyz = 1; last SWITCH; }
$nothing = 1;
}
There is no official switch statement in Perl, because there are
already several ways to write the equivalent. In addition to the
above, you could write
SWITCH: {
$abc = 1, last SWITCH if /^abc/;
$def = 1, last SWITCH if /^def/;
$xyz = 1, last SWITCH if /^xyz/;
$nothing = 1;
}
(That's actually not as strange as it looks one you realize that you can
use loop control "operators" within an expression, That's just the normal
C comma operator.)
or
SWITCH: {
/^abc/ && do { $abc = 1; last SWITCH; };
/^def/ && do { $def = 1; last SWITCH; };
/^xyz/ && do { $xyz = 1; last SWITCH; };
$nothing = 1;
}
or formatted so it stands out more as a "proper" switch statement:
SWITCH: {
/^abc/ && do {
$abc = 1;
last SWITCH;
};
/^def/ && do {
$def = 1;
last SWITCH;
};
/^xyz/ && do {
$xyz = 1;
last SWITCH;
};
$nothing = 1;
}
or
SWITCH: {
/^abc/ and $abc = 1, last SWITCH;
/^def/ and $def = 1, last SWITCH;
/^xyz/ and $xyz = 1, last SWITCH;
$nothing = 1;
}
or even, horrors,
if (/^abc/)
{ $abc = 1 }
elsif (/^def/)
{ $def = 1 }
elsif (/^xyz/)
{ $xyz = 1 }
else
{ $nothing = 1 }