If a user decides to exit while in that second script (which is a child process in my mind), how can I tell the parent to exit as well? And can I tell the difference between a user-selected exit versus the script running its course successfully?

Programs have an exit status. You can use that as a form of signal. Read perldoc -f system to find out how to find the exit status of the called program. Of course this depends on what the called program does on user exit. For anything more complicated you will need some sort of IPC.