SOAP or XML/RPC?
With a bit of hacking, I've been able to keep SOAP::Lite working on a variety of systems over the past few years. I have never gotten XML/RPC to work. YMMV
UPDATE: SOAP::Lite has been updated, it works just peachy with Apache 2.0-2.2, mod_perl 2.0+, and NicTool.
The XML/RPC code yields errors complaining about extra information in the response. I'm sure some someone with experience using RPC::XML could whip it into shape in a hurry, but it's not at the top of my list.