{snip} how two mutually exclusive PERL programs can communicate by sending signals to each other.

Although you probably meant "signals" in the standard Unixy sense, in the more generic sense (and if the Perl programs are running on different machines, and if they don't need to talk to each other very often), I've seen email messages used for inter-app communication -- and it didn't work too badly either. :)

Granted, it's not optimal, and there's often better solutions out there, but it's a hack I've seen done.