SOAP is based on the earlier (but still useful) XML-RPC protocol. the Frontier::RPC modules from CPAN implement it in perl.

Frankly, I think most of this web services stuff is just hype. There seems to be very little you couldn't do with existing methods such as CGI. For an XML application server that integrates with Apache/modperl, I'm looking into AxKit but I don't know enough to really have an opinion on it yet.