Is this a one-time task? If so, can you just do poor-man's multitasking and (for sake of discussion i'll assume you have 3 processors) just start 3 perl processes, each covering a different third of the files to convert?
(though it seems like you're really close w/the much cooler threaded solution -- but just in case it comes down to a time crunch and you have bust out the sledgehammer for the square peg....)