“The difference between a client and a server” has nothing to do with “UDP.”   Rather, it has to do with the role that each program plays, in relation to one another.

A “server” is any computer program that is primarily designed to talk to other computer programs, rather than to people.

“Servers” often work by listening on network connections ... TCP/IP or UDP ... but some internal-only servers use other techniques such as named pipes.   Many commercial servers support multiple modes of communication.

Within CPAN, you will find substantial building-blocks for constructing server programs.   If you look hard enough before “just diving in,” you’ll find that a great deal of your work has been done for you.   The code is, by and large, very high-performance, reliable, and well tested.