In Perl, there are 2 different kinds of hashes.

• A hash, which is defined by the % and ( ) characters - %hash = ( );
• A reference hash, which is defined with the $ and { } characters - $hash = { };

The following if statement will return "is defined" when the hash array named %hash is defined.

if ( %hash ) {
  print "The regular hash named \%hash is defined\n";
}

If the hash array named %hash has not been defined, something like this will be returned.

Global symbol "%hash" requires explicit package name

Let's say you create a regular hash that contains an array. In this example, the foo key is an array, as indicated by the [ ] characters.

my %hash = ( 'foo' => [] );

Dumper can be used to display the structure.

use Data::Dumper;
print Dumper \%hash;

Which should return the following.

$VAR1 = {
          'foo' => []
        };

Now the following if statement should return "is defined".

if ( %hash ) {
  print "is defined \n";
}

Here is how you would determine if %hash contains a key named "foo".

foreach my $key ( keys %hash ) {
  if ( $key eq "foo" ) {
    print "The foo key is defined \n";
  }
  else {
    print "The foo key is not defined \n";
  }
}