You don't because system starts a synchronous child process (e.g. perl starts "PMEMD" and then waits for it to exit before continuing). If you want to create a separate simultaneous child process you need to either use bare fork and exec yourself or look into a module such as IPC::Run or the like.

Update: And before someone else brings it up, yes on *NIX-ish OSen you can append an & to the command line you send to system and get the process backgrounded. But that's just asking for pain (especially since the OP wants the child's pid yadda yadda).