We have a similar challenge with our build system. Our builds are independent of whatever happens to be on CPAN at the moment. We do a lot of testing, and we don't want to introduce CPAN changes or availability as a variable.

We use a perl program that builds all our modules, and it has to manage the dependencies. It uses the dependencies listed in the module as a starting point, but many modules do not list their dependencies correctly, so we have to refine the list by hand. Of course, the dependencies themselves depend on the version of perl, since more modules are brought into the core in higher versions.