Which is to say you can't call a sub in a different process using conventional means, but there are other means of doing so. What the OP asked about is generically called Remote Procedure Call. And RPC isn't even required. The process can also receive more generic work requests (as opposed to function call requests) via a number of means including sockets (Inter-Process Communication).

OP, this is a huge topic. You'll need to specify a lot more information.