It is because of simplicity. IO::Socket provides you with low level protocol. Basically you can just read and write strings from a stream. You will have to roll your own high level protocol for communication between game server and client. You mentioned that you want several different clients. It means that unless they are all written in Perl they all will need their own libraries for handling of this high level protocol.

With SOAP you already have high level protocol which is portable across different OSes and programming languages. Basically it a protocol which allows you to do RPC (function and method calls). SOAP::Lite is very convinient librariy which makes it completly transparent in Perl. See yourself. Read SOAP::Lite Guide.

As for perfomance SOAP::Lite will be probably slower than your own protocol over IO::Socket. But it acceptable price in most cases unless you have very strict perfomance requirements.