We must identify what key can be used to determine which version client program is making the request. In the client request stream is there something that is unique to each version? If there is something unique, then it would not be hard for a perl-based server to intercept the request and reformat into some standard (most recent) version of the actual billing server. The Perl-based front-end would act like a request broker, many-to-one.
in reply to Electronic Billing question - where to start
Is this sort of what you need? An example of the differences
would help clarify your needs.