from DEPRECATED.pod:
=item Perl PMCs
The dynamic PMCS PerlString, PerlUndef, etc. were originally created with an
eye towards being used in Perl6 development, and as the first set of full
featured PMCs. However, Perl6 development is not going to be using them, and
the core PMCs (String, Undef, Integer ...) have replaced them for usage.
These PMCs will be moved to languages/perl5 after the next release. At some
point after that they may be removed the repository altogether.
this is a placeholder ticket for this work, and will be updated in
DEPRECATED.pod as soon as the ticket number is generated.
~jerry