Given a piece of code to parse a single directory, how can I parse multiple directories concurrently (or very nearly) without the memory overhead of each piece requiring its own intrepreter? Concatenating the files in each directory into one long list isn't feasible either.

I'd probably switch to an event-based model - maybe use POE. Threads are probably not what you want in this instance. At the moment Perl's threads are fairly heavy and slow - more like wet string :-)