In this case, if you would do an exec() instead of a meaningless ` ` construct (you're not doing anything with the captured output, in fact, you're redirecting all possible output to a file), you would know the pid, because it would be the value returned by fork().

Also, the open("|") type constructs return the child PID.

update: and loose the & at the end of the syscall, you've already forked off a seperate process.

update2: oh, I forgot. If you do pipes and redirect like you do, you're in fact starting a shell, which starts several other processes. So in fact, the only program started by perl would be the shell.