I'm no expert, but I don't think multicast is what you're looking for. IO::Socket::Multicast lets you send and recieve multicast UDP packets. UDP means no connections and multicast means no control over who receives the messages. The docs for the module do a pretty good job explaining this - did you read them?
You read the documentation and you're still wondering how you can have your server wait for connections from clients? The docs say:
These addresses do not correspond to individual machines, but to multicast groups. Messages sent to these addresses will be delivered to a potentially large number of machines that have registered their interest in receiving transmissions on these groups. They work like TV channels. A program tunes in to a multicast group to receive transmissions to it, and tunes out when it no longer wishes to receive the transmissions.
That's probably the best description of multicast UDP I've read. It's definitely not a client/server system in the usual sense - there's no way a server can wait for connections because there are no connections!