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 have the following hash. In this example, the foo key in the hash is empty.

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

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

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

Which should produce the following.

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

A value can be appended to the foo key, like this.

$hash{"foo"} = "bar";

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

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

Now, the foo key contains a value of bar.

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

Here is how you would append a value (Hello World) to multidimensional keys, where the "bar" key is below the "foo" key.

$hash{foo}->{bar} = "Hello World";

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

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

Now, the bar key contains a value of Hello World.

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