What do you mean by "I cannot invoke CPAN from the shell"? What unexpected behaviour do you see?
I don't recommend mixing RPM-installed modules with CPAN-installed modules in the same Perl installation. It's bound to lead to problems eventually (there's a chance it even led to whatever problems you're currently having).
The system Perl that comes with Fedora already has a number of RPM-installed modules with it. I therefore strongly recommend only installing new modules using RPM or yum. I know that not all CPAN modules are available that way but you have a couple of options. You can find new repositories that have other CPAN modules available (I run one such repository - and I take requests for new modules) or you can learn how to create your own RPMs from CPAN modules - it's really not that hard.
See my presentation Perl in RPM-Land for more details.