I managed to cut the run time in half (thank you all for your help). During this process, the algorithm was changed in a manner such that there is now opportunity for the logic to be implemented in parallel; previously the logic was strictly sequential.

I tried to use threads, but the numerous calls to create the threads created way too much overhead for any speed improvement. My program is not memory intensive; the memory usage maxes out at just over 4MB before the program finishes; yet, by using threads the required memory exceeded the 2GB on my machine.

Dual core machines are very common these days. I was hoping to re-write the program to use both processors. I was thinking of having a service run on one processor (which takes requests to find pairs of numbers) while the main program looks for valid numbers, waits for all the pairs to be found, then finishes the last step of the process.

I'm using windows (it sounds like perl behaves slightly differently on windows). I need some type of inter-process communication mechanism to submit new requests, eventually wait and receive back the (hash table) results. At this point, I don't need detailed explanations, just your insights and recommendations on which modules are best suited to this problem.

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