Perl Control Structures - Conditional Statements

Conditional Statements

if ( expr ) block if ( expr ) block else block if ( expr ) block elsif ( expr ) block ... else block unless ( expr ) block unless ( expr ) block else block unless ( expr ) block elsif ( expr ) block ... else block

where block is a sequence of one of more Perl statements surrounded by braces.

The controlling expressions are evaluated in a boolean context: The numeric value 0, the strings "" and "0", and the undefined value undef are false, all other values are true. This means that the strings "0.0", "00", "-0", and "0 but true" are all true, even though their value would be converted to 0 in a numeric context; values like these are sometimes used when a successful operation needs to return 0.

Evaluating an empty array or hash in scalar context yields undef, which is false. Therefore, the following example prints "a is empty":

my @a=; unless (@a) { print "a is empty" }

Read more about this topic:  Perl Control Structures

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