Is there a simple way to use the builtin/base perl but use pkgsrc
otherwise? 

I'm guessing that because there is no lang/perl5/builtin.mk the answer
is NO.

I believe that a shell of a pkg which satisfies the pkg_info state, but
has symlinks back to the /usr/lib/perl5/... will probably work. Before
I do anything that disgustingly hackish, I thought I'd ask what others
do.

-George