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 = { };

Let's say you a hash named %hash that will be used to store employees data. In this example, the employees key is an array, as indicated by the [ ] characters and the employees key is empty (contains to keys or values). Dumper can be used to display the structure of the hash.

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my %hash = ( 'employees' => [], 'manager' => [] );

print Dumper \%hash;

Which should produce the following.

$VAR1 = {
          'employees' => [],
          'managers' => []
        };

Let's say you do this.

push @{$hash{employees}}, { 'name' => '', 'department' => '' };
push @{$hash{managers}}, { 'name' => '', 'department' => '' };

Now, the hash should look like this.

$VAR1 = {
          'employees' => [ 
                           {
                             'name' => '', 
                             'department' => ''
                           }
                         ],
          'managers' => [ 
                           {
                             'name' => '', 
                             'department' => ''
                           }
                         ]
        };

You can loop through the hashes and use Dumper to print only the keys.

foreach my $employee (@{$hash{employees}}) {
  foreach my $key ( %$employee) {
    print "\$key = $key \n";
  }
}

Which should return the following.

$key = name
$key = department