I think you're going to have to take a different tack on this one. E.g. If you have Cisco Catalysts, you might be able to poll them for their CAM tables, and pull back connected info from that, but the best way to get these addresses is probably by polling a hardware information program resident on each PC.

Net::Address::Ethernet essentially returns the MAC address of physically installed ethernet cards on the machine. Not on other machines. If you can get the program to run on the other machines remotely, then you might be able to poll them all. Do you have some bulk distribution mechanism for the PCs?

Wish I could be more help,
-v

Update: see monarch's post, below, for excellent info about SNMP polling and segment restrictions.