I agree with all the other replies to this - all valid ways of using Perl to check for this open port, but...

Why are you so concerned about RAdmin in particular? I know from experience that there are thousands of programs around that will quite happily drop a screen capture/keylogger on your machine. Of course, these all run on a myriad of different ports - not to mention the fact that if I wanted, I could change RAdmin's listening port and therefore circumvent your checks.

The other big killer on an exploited Windows box (much like *NIX) is not to trust any console tools - most backdoor writers these days are savvy enough to hide their processes by running a kernel layer filter. These are most commonly used to hide themselves from task manager, but I wouldn't put it past them to hide their network ports either.

It's not that any of these suggestions are wrong in the Perl sense, and forgive me if I'm preaching to a learned monk, but it might just lull you into a false sense of security. A good software based firewall (hell, even XP SP2's inbuilt one) are going to cover most of the bases in this area.