In my research I actually found some packages that allow TCP/UDP socket read and write. I would imagine that in many cases, when one application must read a stream from another, UDP is a good solution. I didn't go that route since I couldn't imagine asking my end-users to make sure their firewall is open on port XXX for what is, for the user, a single autonomous application.

Right now, I'm modifying the architecture or this little application so the main script "gui.pl" will call the second one by importing its subroutine(s), not by executing it. So far it's not many modifications. WIP...

I appreciate all the effort people have put in my question :)