If you are concerned about size and memory consumption then I'd suggest looking at DB_File, MLDBM and Storable. You can then tie your hash to the disk. However, you have shifted your memory problems to disk problems. But generally speaking HDD space is cheaper than memory space.
However, as has been mentioned above, Perl can handle very large sets of data quite easily.
I do that for another implementation but it seems to be
very slow. I want the world! But actually, I've decided to stick with the hash until the data gets
too big, then I'll probably do a DB_Hash.