First of all, there is dev-lang/perl. This contains the Perl core distribution, installing the binaries and all the Perl modules that are bundled with Perl itself.
Then, there is the perl-core category. It contains independent ebuilds for Perl modules that are also present in the core Perl distribution. Most Perl modules that are bundled with Perl are also in addition released as independent tarballs. If any of these packages is installed from perl-core, its files are placed such that the perl-core download overrides the bundled copy. This means you can also update part of the bundled Perl modules, e.g. in case of a bug, without updating Perl itself.
Next, there are a lot of virtuals "virtual/perl-..." in the virtual category of the portage tree. What are these good for? Well, imagine you want to depend on a specific version of a module that is usually bundled with Perl. For example, you need Module::CoreList at at least version 3. This can either be provided by a new enough Perl (for example, now hardmasked Perl 5.20 contains Module::CoreList 3.10), or by a separate package from perl-core (where we have Module::CoreList 5.021001 as perl-core/Module-CoreList-5.21.1).
To make sure that everything works, you should never directly depend on a perl-core package, but always on the corresponding virtual (here virtual/perl-Module-CoreList; any perl-core package must have a corresponding virtual). Then both ways to fulfil the dependency are automatically taken into account. (Future repoman versions will warn if you directly depend on perl-core. Also you should never have anything perl-core in your world file!)
Last, we have lots of lots of modules in the dev-perl category. Most of them are from CPAN, and the only thing they have in common is that they have no copy inside core Perl.
I hope this clarifies things a bit. More Perl posts coming...