You might want to enable arpwatch on your server and then look in to arp.dat. I'm not really sure as to the format of the file. However looking in to the file with perl may be unnecessary as arpwatch itself notifies you through email of any changes.

Here's an excerpt from the man page.

Arpwatch keeps track for ethernet/ip address pairings. It syslogs activity and reports certain changes via email.

This will keep track as to what machines you have. As far as monitoring them to make sure they're up or down. I would suggest using Big Brother rather than reinventing the wheel.

BMaximus