The same parameters should be passed on correctly, unless you've done something to muddle with @ARGV. I'd rewrite the system command to be a little more clear though:

system("$RUN_PROCx @ARGV");
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Looks a lot nicer, and you don't have to worry about adding an $ARGV[6] should the need arise. As far as using the same PID number from process-to-process, those are assigned by the OS and there's no real way to coerce those. (If you're just passing them around, use them as an additional argument.) IPC::* is probably overkill.