Are you running ActiveState perl? If you are, you should try re-installing libnet (instructions below). I've found that most of the libnet modules don't work well out of the box from ActiveState.

To install libnet:
1. run 'ppm'
2. type 'install libnet'
3. answer 'y' to when it askes you if you want to install
4. after a bit, it will ask you if you want to install again (it detected a previous version) answer yes
5. you can accept the defaults from here on out (unless you want to change something)