mysql would be MUCH faster than a flat text file. MySQL itself wouldn't be doing the work to parse the data, you'd store it there then extract it with perl.

As far as passing variables i'm thinking that each engine is going to take it as a parameter, not a variable name, so the variable is only going to hold the data, not the name. I'm a little stumped as to what exactly you're having trouble with there though...

UPDATE: I thought about it a bit, and i've found myself in this situation before. My advice would be to study each of the engine you are submitting to and figure out which values each of them take in their form submissions. "View Source" is your friend :-)