If you add an ampersand after the command system() will fork off the process for you and return immediately. However, before you do anything more you should read perlipc. There's a lot of good information there that applies to the kind of script you're writing. If you have questions after reading it come back and ask them here.

UPDATE: You might also want to take a look at PSMON. If you can't use it directly I'm sure you could learn a lot from reading the source. It's a Perl script which monitors processes and takes actions based on configurable rules.

-sam