Perl unless statement represents an alternative to the conditional Perl if statement. Regardless the if statement which executes a block of code if a condition is true, the unless statement executes a block of code if the condition is false.

We can regard the unless statement like the negative or opposite of if. Sometimes it is useful to use unless instead of if, especially when you want to optimize your code or to make it more legible.

Because if and unless are very similar, we’ll not insist too much on this statement. This short tutorial will show you the main syntax forms of the Perl unless statement and it will point out some examples of how to use it in your script applications.

I think it is not a bad idea for you to consult my page dedicated to Perl if statement, first. You can use Perl unless statement either in connection with else and elsif clauses if your conditional statement has more than one branch, or as a modifier.

to see a lot of fully commented examples that help you use the Perl statements and the Perl buit-in functions in your scripts.

STATEMENT unless (EXPR)

If the EXPR condition is true, the Perl interpreter will skip to the following line of code. You can use this form when your conditional statement has no more than one branch. See the next example about how to use it:

# define an array
@colors = qw(blue green brown cyan white);
print "@colors\n" unless scalar(@colors) < 5;
# prints: blue green brown cyan white

By enclosing @colors array between double quotes, we tell Perl interpreter to interpolate the array and print its elements separated by space.

unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK

• EXPR represents a Boolean conditional expression
• elsif and else clauses are optional and are used if you want to check a certain sequence of conditional expressions and find out which one of them is true
• BLOCK consists of one or more statements enclosed in curly braces; in the construction of a block, the curly braces are always required.

unless (EXPR) BLOCK

$b = 0;
unless($b != 0) {
  $b += 1;  
}
print "b = $b\n";  # it outputs b = 1

to see a lot of fully commented examples that help you use the Perl statements and the Perl buit-in functions in your scripts.

unless (EXPR) BLOCK else BLOCK

Here is an example that finds the minimum of two numbers:

$a = 35;
$b = 22;
unless($a > $b) {
  print "min($a, $b) = $a\n";
} else {
  print "min($a, $b) = $b\n";
}
# it prints: min(35, 22) = 22

With the ternary operator, we can rewrite the above code like this:

$a = 35; $b = 22;
print "min($a, $b) = ", $a < $b ? $a : $b,"\n";

unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK

$a = 12;
unless($a >= 0) {
  print "a is negative\n";
} elsif ($a == 0) {
  print "a is equal to 0\n";  
} else {
  print "a is positive\n";  
}
# it prints: a is positive

As you have seen so far, the Perl unless statement is the reverse of the Perl if statement and you can use unless instead of if and vice-versa as you wish, in order to make your code more readable.

to see a lot of fully commented examples that help you use the Perl statements and the Perl buit-in functions in your scripts.