DB_File can not store complex data structures as values, only simple scalar values (i.e. strings, numbers) for each hash key. Therefore you can not store a hash of arrays of hashes. Unless you can serialize your value in some way (e.g. create a delimited string value out of the array of hashes that you're trying to store for each value), then Storable is the way to go.

Update: Oh, neat, I didn't know about MLDBM, you learn something all the time around here :)