First of all, you're very close-the function you may wish to look at, though, is grep(), because you can do something like print OUTPUT_A join ( ',', $fn, $ln, $id ), "\n" if ( grep( /$id/, @a ) ); with it. I can't see a really good reason for changing to hashes for the IDs at the moment.

Secondly, and more importantly, don't beat yourself up about this. You weren't that far off-you just needed a function you hadn't encountered yet. Yes, it can be flustrating sometimes, but never give up. Perhaps you may want to spend some time once in a while browsing here or the function listing. It sounds like you have a reasonably good starting point with the resources you currently have-just browse through them at times, or take one of the examples and see what you can do with it, or try something and if it doesn't work, feel free to ask here or in the CB.