Having a hash instead of grepping is of course better. (although it takes more memory)

The idea of using the memory reference returned by perl's internal heap mechanism is interesting, but i'm not sure it buys much here.

Anyway, the original post is "walking on the edge" of usability. If his files are much bigger than the computer memory, then the hash will not fit in and then there are better ways, such as using database, doing multiple passes etc. (or keeping the files clean in the first place...)

Cheers.