Switch statements

A BLOCK by Perl (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. (Note that this is NOT true in eval{}, sub{}, or contrary to popular belief do{} blocks, which do NOT count as loops.) The continue block is optional.

The BLOCK construct can be used to emulate case structures.

SWITCH: {
   if (/^abc/) { $abc = 1; last SWITCH; }
   if (/^def/) { $def = 1; last SWITCH; }
   if (/^xyz/) { $xyz = 1; last SWITCH; }
   $nothing = 1;
}

Alternatively you'll also emulate switch using the  foreach loop with scalar variable instead of array:

SWITCH:
for ($var) {
  if (/^abc/) { $abc = 1; last SWITCH; }
  if (/^def/) { $def = 1; last SWITCH; }
  if (/^xyz/) { $xyz = 1; last SWITCH; }
  $nothing = 1;
}

Such constructs were sometimes  used because older versions of Perl had no official switch statement

Starting from Perl 5.10.1, Perl has special switch feature. You can get it is you use use v5.10.1 or later.  For example use v5.16.3; (default in RHEL7) 

NOTE  "switch" feature implements intellectual comparison (~~) in when. This "intellectual comparison" is considered highly experimental; it is subject to change with little notice. Due to this  when has tricky behaviors that are expected to change to become less tricky in the future. Do not rely upon its current (mis)implementation.

Before Perl 5.18, given also had tricky behaviour that you should still beware of if your code must run on older versions of Perl. It is better to avoid it.

    use v5.10.1;
    for ($var) {
        when (/^abc/) { $abc = 1 }
        when (/^def/) { $def = 1 }
        when (/^xyz/) { $xyz = 1 }
        default       { $nothing = 1 }
    }

The for loop with scalar assigns the $_ variable that can be reused in when.  You can also use keyword given, but it looks redundant and unnatural in this context. For is preferable.

As of 5.14, that can also be written this way:

use v5.14;
for ($var) {
   $abc = 1 when /^abc/;
   $def = 1 when /^def/;
   $xyz = 1 when /^xyz/;
default { $nothing = 1 }
}

This construct is can make code more clear. For example:

    use feature ":5.10";
    given($foo) {
        when (undef) {
            say '$foo is undefined';
        }
        
        when ("foo") {
            say '$foo is the string "foo"';
        }
        
        when ([1,3,5,7,9]) {
            say '$foo is an odd digit';
            continue; # Fall through
        }
        
        when ($_ < 100) {
            say '$foo is numerically less than 100';
        }
        
        when (\&complicated_check) {
            say 'complicated_check($foo) is true';
        }
        
        default {
            die q(I don't know what to do with $foo);
        }
    }

given(EXPR)  will assign the value of EXPR to $_  within the lexical scope of the block, so it's similar to

        do { my $_ = EXPR; ... }

except that the block is automatically broken out of by a successful when  or an explicit break.

Most of the power comes from implicit smart matching:

        when($foo)

is exactly equivalent to

        when($_ ~~ $foo)

In fact when(EXPR)  is treated as an implicit smart match most of the time. The exceptions are that when EXPR is:

o

a subroutine or method call

o

a regular expression match, i.e. /REGEX/  or $foo =~ /REGEX/, or a negated regular expression match $foo !~ /REGEX/.

o

a comparison such as $_ < 10  or $x eq "abc"  (or of course $_ ~~ $c)

o

defined(...), exists(...), or eof(...)

o

A negated expression !(...)  or not (...), or a logical exclusive-or (...) xor (...).

then the value of EXPR is used directly as a boolean. Furthermore:

o

If EXPR is ... && ...  or ... and ..., the test is applied recursively to both arguments. If both arguments pass the test, then the argument is treated as boolean.

o

If EXPR is ... || ...  or ... or ..., the test is applied recursively to the first argument.

These rules look complicated, but usually they will do what you want. For example you could write:

    when (/^\d+$/ && $_ < 75) { ... }

Another useful shortcut is that, if you use a literal array or hash as the argument to when, it is turned into a reference. So given(@foo)  is the same as given(\@foo), for example.

default  behaves exactly like when(1 == 1), which is to say that it always matches.

See "Smart matching in detail" for more information on smart matching.

Breaking out

You can use the break  keyword to break out of the enclosing given  block. Every when  block is implicitly ended with a break.

Fall-through

You can use the continue  keyword to fall through from one case to the next:

    given($foo) {
        when (/x/) { say '$foo contains an x'; continue }
        when (/y/) { say '$foo contains a y' }
        default    { say '$foo contains neither an x nor a y' }
    }

Switching in a loop

Instead of using given(), you can use a foreach()  loop. For example, here's one way to count how many times a particular string occurs in an array:

    my $count = 0;
    for (@array) {
        when ("foo") { ++$count }
    }
    print "\@array contains $count copies of 'foo'\n";

On exit from the when  block, there is an implicit next. You can override that with an explicit last  if you're only interested in the first match.

This doesn't work if you explicitly specify a loop variable, as in for $item (@array). You have to use the default variable $_. (You can use for my $_ (@array).)

Smart matching in detail

The behaviour of a smart match depends on what type of thing its arguments are. It is always commutative, i.e. $a ~~ $b  behaves the same as $b ~~ $a. The behaviour is determined by the following table: the first row that applies, in either order, determines the match behaviour.

    $a      $b        Type of Match Implied    Matching Code
    ======  =====     =====================    =============
    (overloading trumps everything)

    Code[+] Code[+]   referential equality     $a == $b
    Any     Code[+]   scalar sub truth         $b->($a)

    Hash    Hash      hash keys identical      [sort keys %$a]~~[sort keys %$b]
    Hash    Array     hash slice existence     grep {exists $a->{$_}} @$b
    Hash    Regex     hash key grep            grep /$b/, keys %$a
    Hash    Any       hash entry existence     exists $a->{$b}

    Array   Array     arrays are identical[*]
    Array   Regex     array grep               grep /$b/, @$a
    Array   Num       array contains number    grep $_ == $b, @$a
    Array   Any       array contains string    grep $_ eq $b, @$a

    Any     undef     undefined                !defined $a
    Any     Regex     pattern match            $a =~ /$b/
    Code()  Code()    results are equal        $a->() eq $b->()
    Any     Code()    simple closure truth     $b->() # ignoring $a
    Num     numish[!] numeric equality         $a == $b
    Any     Str       string equality          $a eq $b
    Any     Num       numeric equality         $a == $b

    Any     Any       string equality          $a eq $b


 + - this must be a code reference whose prototype (if present) is not ""
     (subs with a "" prototype are dealt with by the 'Code()' entry lower down)
 * - that is, each element matches the element of same index in the other
     array. If a circular reference is found, we fall back to referential
     equality.
 ! - either a real number, or a string that looks like a number

The "matching code" doesn't represent the real matching code, of course: it's just there to explain the intended meaning. Unlike grep, the smart match operator will short-circuit whenever it can.

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