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 = { };

The first step in creating a hash that contains an array is to define a hash or define a reference hash that has at least one key that contains an array. 

Hash

Here is how you would define an empty hash named %hash where the foo key contains an array. The [ ] characters are used to set the foo key as an array.

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

Or like this.

my %hash;
push @{$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' => []
        };

Reference Hash

Here is how you would define an empty reference hash named $hash where the foo key contains an array. The [ ] characters are used to set the foo key as an array.

my $hash = { 'foo' => [] };

Or like this.

my $hash;
push @{$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' => []
        };

Now you can append values to the hash array.