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, %hash is empty (no keys, no values).

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 = {};

Here is how you can append key value pairs to the hash.

$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'
        };

Let's say you do this.

$hash{foo} = "hello";
$hash{foo} = "world";

In this scenario, the foo key will contain world. This overwrites the value in the foo key.

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

One option might be to use different keys.

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

Which should produce the following.

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