Hmm: This is not really a Perl question...

You could run some network scanning tool such as nmap. I got the following from overclockers.com:

which runs a very aggressive test on all machines and gives you a lot of information such as Vendor, OS, MAC, etc. On my machine you need to run this as root. Unless you own the network, this might be seen as an aggression.

UPDATE: As always, you can probably do it within Perl as well as there is an Nmap::Scanner module in Perl which might achieve similar feats...