It's a long time since I had to configure cpan (well, I did it today when I ran dh-make-perl for the first time on a new machine, but it doesn't count). Usually for system perl I just use packages from the repository, or build packages with dh-make-perl if the repository doesn't have what I need. For perlbrewed perls I don't have to care about locations, so I just use cpanm without any configuration. Perlbrew may also help with Client-X/Y thing, you can just install new perl for every project, thought I don't have such a problem.