You’ll find out several interesting ways to replace the cascaded ifs by the switch pattern and you’ll learn some useful techniques that you can use in your conditional statements. Please have a look at the next simple pseudo code:

if(variable == 1) {
  statement_1";
} elsif(variable == 2) {
  statement_2";
} elsif(variable == 3) {
  statement_3";
} else {
  default_statement";
}

In some languages like PHP or C, there is the switch statement that let you rewrite the above pseudo code as follows:

switch(variable) {
  case 1:
    statement_1";      
    break;
  case 2:
    statement_2";
    break;
  case 3:
    statement_3";
    break;
  default:
      default_statement;
}

Now that you know the meaning of the switch statement, the bad news is that there is no Perl switch/case statement in the versions prior to 5.8.x, because of the huge variety of tests available: numerical and string comparisons, regexp matching, overloaded comparisons and so on.

The good news is that you can:

1. easily emulate or build it if you work in an earlier version of Perl (prior to 5.8.x)
2. use the Switch module if you work in Perl 5.8.x or
3. use the given/when (instead of switch/case) statement if you work in Perl 5.10 version

In the following I’ll review each of these three opportunities.

to see a lot of fully commented examples that help you use the switch statement in your scripts.

1.1. You can use a bare block in order to construct a case structure that looks syntactically similar with the switch statement found in other languages. We’ll label our block with the noun SWITCH and because a bare block is a loop that executes only once, we can use the last, next and redo loop controls inside. Our compound Perl switch statement could look as follows:

$var = 2;
SWITCH: {
  $var == 1 && do { print "\$var = 1\n"; last SWITCH; };
  $var == 2 && do { print "\$var = 2\n"; last SWITCH; };
  $var == 3 && do { print "\$var = 3\n"; last SWITCH; };
  print "\$var is not equal with 1 or 2 or 3\n";
}
# the output displays $var = 2

1.2. Next I’ll mention another sample of emulated Perl switch statement that uses the corresponding form of if as a modifier, inside a while loop:

print "Gess the colours:\n";
LINE: while(<STDIN>) {
  SWITCH: { 
    $color = "blue", last SWITCH if /blue/i; 
    $color = "white", last SWITCH if /white/i; 
    $color = "red", last SWITCH if /red/i; 
    print "Try again\n"; next LINE;
  } 
  print "color = $color\n";
}

1.3. Another alternative is to use the foreach statement (or its for equivalent name) as in the following construct:

# assign a string value to a scalar variable
$choice = "blue";
SWITCH: foreach($choice) {
  /blue/i  && do { print "Colour is blue\n"; last SWITCH;};  
  /yellow/i  && do { print "Colour is yellow\n"; last SWITCH;}; 
  /green/i  && do { print "Colour is green\n"; last SWITCH;};    
  die "Unknown colour: $choice \n"; 
}
# it displays: Colour is blue

# initialize some hash structure
%fruitsColors = (
  "apple"  => "red",
  "banana" => "yellow",
  "orange" => "orange",
  "plum"   => "dark blue"
);

print "Fruit: ";
chomp($fruit = <STDIN>);
$color = $fruitsColors {lc $fruit} || "unknown";
print "The $fruit has the colour $color\n";

• will read from stdin a word representing the name of a fruit
• will discard the trailing newline of the word (by applying the builtin chomp function)
• next, it will look up in the %fruitsColors hash for the fruit name and if it finds it, will store the given color of the fruit in the $color scalar variable, otherwise the $color variable will be initialized with the text "unknown"
• finally, the result of the search will be printed

to see a lot of fully commented examples that help you use the switch statement in your scripts.

This module was made available beginning with the 5.8.x version of Perl and covers a lot of possible combinations of switch having as single input argument (in a scalar context) a number, a string, a specific pattern, a reference to an array or a hash (associative array), or a call to a subroutine. Like the switch from the C language, the Perl Switch module allows you the use of fall-through – i.e. trying another case after one that has already succeeded.

Now this module was included in the Perl core distribution, but if you have an older version of Perl it’s possible that this module is not included (in this case if you want to use the Perl Switch module you need to install it - on a Windows platform, if you installed ActiveState’s ActivePerl, you can run their Perl Package Manager from your command shell - or upgrade your Perl version). For a full description of all its features, you can locate this module by searching at CPAN after the word 'switch' and learn more ways to invoke it.

The next simple script sample shows you one of the way through which you can use the Perl Switch module in your Perl programs or applications:

use Switch;

$i;
chomp($var=<STDIN>);

switch ($var){
  case(1) { $i = "One"; }
  case(2) { $i = "Two"; }
  case(3) { $i = "Three"; }
  else    { $i = "Other"; }
}

print "case: $i\n";

use Switch 'Perl5', 'Perl6';

Based on the Perl 6 proposal, starting from Perl 5.10 is enabled a new switch feature. This Perl switch statement is spelled given, and case is pronounced when. The using of the given/when statement is very similar (there are minor differences) with the Perl switch/case statement. For instance, the Perl switch statement from the previous example can be rewritten as follows:

given ($var){
  when(1) { $i = "One"; }
  when(2) { $i = "Two"; }
  when(3) { $i = "Three"; }
  default { $i = "Other"; }
}

to see a lot of fully commented examples that help you use the switch statement in your scripts.