Firewalls are pretty low level (ie kernel) code so Perl is not the best language for it. Have a look at http://www.codeproject.com/internet/drvfltip.asp for one way to implement a firewall (there are several) with code and explanations.

Now when it comes to AIM overuse you could consider yourself lucky. Back when I was a young one I seem to remember tying up the phone for hours at a time talking to friends.