I think the socket approach is likely the best, and most documented. Other possiblities include named pipes, and shared memory which I would avoid for your needs here.

Basically, I would say to set up your child process to listen on a defined port and the set up the parent process to send instructions to that port. Is there any reason something like that wouldn't work?

Here is a good node on setting up a client/server setup using IO::Socket. It helped me get off the ground.