This is probably more of an OS problem than a perl problem. That is, you first need to figure out how to determine which user is using your modem. Under a Unix-like system there are OS calls that can be made to figure this out (commands like fuser and ps), but I'm not sure what facilities exist for WinXP.

Secondly, you might be better off just patching your OS and software so the guy can't break in. You might have re-install Windows and all of your apps, but you would be solving a lot of problems all at once by making your system secure. And even if you do figure out the modem thing, what will prevent the hacker from creating other mischief if you don't close up the security hole that's allowing him into your box in the first place?

By the way, when you say "modem", are we talking about a telephone modem (and not a cable or DSL modem)? And is the hacker coming in through your broadband connection to use your telephone modem? to send spam?