g-cpan refuses to install (all?) packages as they are in category 'perl-gcpan', which is unknown to the system. This seems a pretty old issue (https://forums.gentoo.org/viewtopic-t-569126-highlight-gcpan+category.html), so maybe I am missing something... As in URL above, # echo perl-gcpan > /etc/portage/categories solves this issue. Should it be done at g-cpan installation?
Meanwhile, a mention of this could be added to the Wiki page https://wiki.gentoo.org/wiki/Project:Perl/g-cpan
I'm not sure if we should done this during installation. Are there examples of this? And yes, adding note to postinst message and wiki will help.
I've analyze a code little and found what g-cpan should manage this. Seems like it was solved a long time ago. I've tested that by remove/empty /etc/portage/categories and all fine after first run of g-cpan. Could you please provide more details for the case (with logs).