g-cpan which generates ebuilds for perl modules on CPAN tries to generate ebuilds for some modules already included in the perl core distribution when trying to install some modules. if collision-protect is enabled those then fails to install since they try to overwrite the files in the core perl distribution. For example generating ebuilds for Digest::Whirlpool will make ebuilds for Pod::Parser and Digest even thought perl-core/digest-base and perl-core/PodParser already exists, because the names of the generated ebuilds are not exactly the same. I have seen this happen with other perl modules too, both things that are aviable in the perl-core category and things that is included when you install the normal perl ebuild.
Which version of g-cpan are you working with? Latest rc2 of 0.15 generates the following DEPEND list: DEPEND="dev-lang/perl"
This was fixed in two ways in 0.15_rc3 - first, we dep on dev-lang/perl directly, and secondly (and the real fix), when looking for a module, we first check for an ebuild, failing that we check to see if it was installed in the core perl install, and only on failing those two checks do we query cpan. so this should be all set now.
