This is one of 4 packages which are still in my local overlay and which seem to be forgotten by the recent "perl-5.20" conversion: corelist sugggests that perl-5.20 contains digest-base-1.170.0, but none of the ~virtual/perl-digest-base-1.170 ebuilds is satisfied if perl-5.20 is installed.
https://wiki.gentoo.org/wiki/Project:Perl/Ongoing_tasks#Virtuals_that_still_need_some_attention Work in progress