You should have a look at HASHES OF HASHES!

The essential difficulty in perl is, you can represent the same datastructure in a reference form (eg \%Hash) and in (what I call) a list form (eg %Hash). For nesting you need references, because lists are flattened:

(1,(2,3)) is nothing else then (1,2,3)

but (1,[2,3]) is a list with a literal reference to an array as second element.

As mentioned above better also have a look into perlreftut