Assuming you have read the standard Perl documentation on such things (perllol) SoPW is a really good resource for test exercises. Simple Super Searchs for HoH, HoA, HoHoH, ..., get you a stack of nodes like Converting a HoH to an AoA, Comparing HoH and HoHoH and Hashes of hashes to cut your teeth on. Classic test exercises with answers at the back of the book stuff :-D.