In Perl, there are 3 different kinds of hashes.

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

Let's say you do the following. In this example, the name of the hash is %hash and the hash contains a key named employees_key. employees_key is an empty array, as indicated by the [ ] characters.

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

You could use Dumper to display the structure.

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

employees_key is empty.

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

Multiple arrays

Let's say you do this.

my %hash = ( 'employees_key' => [], 'departments_key' => [] );

Now the hash contains two empty arrays.

$VAR1 = {
          'employees_key' => [],
          'departments_key' => []
        };

Multidimensional arrays

Let's say you do this.

push @{$hash{employees_key}}, { 'departments_key' => [] };

Now the hash contains two arrays, the employees_key array and the departments_key array, where the departments_key array is a child of the employees_key array. The departments_key array is empty. However, the employees_key array is not empty, since the employees_key array contains departments_key.

$VAR1 = {
          'employees_key' => [ 
                              { 
                               'departments_key' => []
                              }
                             ]
        };

Let's say you do this.

push @{$hash{employees_key}}, { 'main' => { 'departments_key' => [] } };

Now, below employees_key array is the main_key hash, and below the main"_key hash is the departments_key array.

$VAR1 = {
          'employees_key' => [ 
                              {
                               'main_key' => {
                                              'departments_key' => []
                                             }
                               }
                              ]
        };

Don't do this

Let's say you do this.

push @{$hash{employees_key}}, "departments_key";

Now the employees_key array contains a value. The value is departments_key.  This is outside of the scope of creating an empty array. Instead, this is how you append values to an array.

$VAR1 = {
          'employees_key' => [ 
                              'departments_key'
                             ]
        };