One thing you could do to scale up would be instead of using many different hashes, use a single multi-level hash... so you would do $phrase{$word_one}{$word_two}{$word_three}{$word_four}, that should save some space and get you all your phrases, though it would be more work to get them out.

Then again, all the added hash overhead might nullify the savings... it would make the data more usable in certain applications, however.