It depends on what the processes are doing; if they're mostly sleeping while waiting for results to become available, resource use will be minimal. If they're all trying to do a lot of work at the same time, they may overwhelm the CPU, and you'll have to be careful to throttle them a bit. Parallel::ForkManager can be helpful with that sort of thing.

I just glanced through the MySQL docs, and I didn't see any kind of asynchronous API. You may get some help on the MySQL mailing list; there might be a clever hack that will allow this.