perl-core/Test-Simple/Test-Simple-0.62.ebuild needs perl-core/ExtUtils-MakeMaker, but in DEPEND, there's no perl-core/ExtUtils-MakeMaker.
I don't think it depends on that specific ebuild, but rather on that specific module. And since this module is included in the Perl distribution (see `perldoc perlmodlib`), I don't think a dependency should be added. A fact is that I just emerged perl-core/Test-Simple without ever emerging perl-core/ExtUtils-MakeMaker. Just my $.02...
Like Yuval said - this isn't an oversight. The ebuild for MakeMaker is for those folks (or at least it was) that were running ancient perl versions but needed to be able to install current perl modules (and so the eclass didn't have to have different build syntax for every version of makemaker out there since 5.6). This is not a bug or an oversight, it is intentional. Your perl install came with MakeMaker, that's all you need.