I am not sure why you would need a DHCP server in perl? One easy way to determine the interface of an incoming packet is to use iptables with the LOG target. A rule similar to the following (untested) should log udp packets coming through eth0 to your /var/log/messages

iptables -I INPUT -p udp --dport 67 -i eth0 -j LOG \
--log-prefix \
"broadcast on eth0"
[download]

I do not know if what you want can be done using existing perl modules. Your problem requires querying kernel info, and am not sure if there are perl modules for that.