Well, it doesn't have to lose any data you need to keep - I originally wrote it as $hash->{$key} = 1, and changed it to an increment to preserve the count; it could just as easily store an arrayref of line numbers or file positions for each key. The core concept though is that the primary super-slow operation being performed is searching through a file repeatedly for matches on a key, and if there is any way to turn that into a hash lookup it's likely to improve things as long as there is memory left.

I think we'd need to see some information about the actual task at hand to know whether there's some reason an in-core hash cannot be used, but unless memory is unusually restricted for a modern-day computer it seems unlikely.

Hugo