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 = { };

Here is how you can create an empty reference hash.

my $hash = {};

Dumper can be used to display the structure of the hash.

use Data::Dumper;
print Dumper $hash;

Which should produce the following.

$VAR1 = {};

Keys that contain no values (empty keys) can be created like this.

my $hash = { 'foo' => '', 'bar' => '' };

Or like this.

$hash->{foo} = undef;
$hash->{bar} = undef;

Or like this.

$hash->{foo} = '';
$hash->{bar} = '';

Dumper can be used to display the structure of the hash.

print Dumper $hash;

Which should return the following.

$VAR1 = {
          'bar' => '',
          'foo' => ''
        };

Let's say you want to create a reference hash that has two (or more) keys, like this. In this example, the bar key is a child of the foo key.

'foo' {
        'bar' => 'Hello World'
      }

Here is how you can create the multidimensional keys and value.

my $hash = { 'foo' => { 'bar' => 'Hello World' } };

Or like this.

my $hash->{foo}->{bar} = "John Doe";

Dumper can be used to print the hash.

print Dumper \%hash;

Which will produce the following.

$VAR1 = {
          'foo' => {
                     'bar' => 'Hello World'
                   }
        };

Or to print the value of the bar key.

print $hash->{foo}->{bar};

Which will produce the following.

Hello World