Simple: open a pipe to ssh
Or run your Perl programs in a Web server, and write a PHP HTTP client for them (fopen, curl).
If that is not possible or sufficient, then you're looking for RMI/distributed computing (or conceptionally equivalent). If you have no idea where to start, simply run a Perl SOAP server and PHP SOAP client, SOAP sucks hairy donkey balls but at least it's painless in PHP. There are many more protocols (XML-RPC, JSON-RPC, Corba, …) and you can invent your own protocol (on top of sockets, on top of HTTP, …).