I also caveat that I've never used this module before. You could try specifying the interface device rather than the IP assigned to it, but since mcast_add just uses that to lookup the device IP, I can't see that it would make any difference.
in reply to Same multicast packet received by multiple NICs
If I want to see what's appearing on the NIC, I lean towards going to the source. You can use Net::Pcap with a compiled filter to get what you're looking for -- no pesky network stack mucking things up :)