It's actually a hash of arrays. That is, it is like a standard hash, but instead of the values being scalars, they are array references:

my %data = (
    'abacus' => [ 1, 2, 3, 4, 5],
    'qwerty' => [ 5, 4, 3, 2, 1]);
[download]

This way, you can refer to the arrays by name without having to use symbolic references (you can store array names in variables, and use those variables as keys to the hash -- that will prevent some subtle errors).

See perlref and perldsc for more information. It's a very powerful technique.