That depends on what you mean by "calling each other". It could be as simple as "system", if you want to call one program from the other. Or you might want to communicate over a pipe, socket or message queue. Or exchange information using files or databases. Or do you want to call procedures in each other? Then you might want SOAP or even RPC. Or do you want to embed one in the other? That's possible too - Parrot isn't quite ready yet, but go read "man perlembed".