One approach is embedding the usual perl interpreter via libperl.

jnthn's blizkost enables Perl 5 code to be executed within Parrot (and hence using Rakudo on Parrot). I think jnthn recently said that blizkost is currently broken and that he hopes to fix it in the next few weeks.

Aiui, blizkost currently does very little, just eval'ing a single line of perl5 code, and not even making use of the result. I think that developing it toward supporting Perl 5 modules was blocked on other progress, in particular implementing native 6model. Maybe it'll move forward this year.

Update August, 2014: jnthn implemented native 6model in 2012/2013 -- in a brand new VM MoarVM, not Parrot. nine (Stefan Seifert) then started Inline::Perl5 at YAPC::EU 2014. Within a couple days this new NativeCall based approach had already established itself as a great approach for delivering P5 interop, once P5 object destruction has been dealt with. It looks likely to me that this project will assimilate aspects of diakopter's unimplemented proposed P5 Interop design which in turn had assimilated blizkost.

Alternatively, once Perl 6 is sufficiently complete, it might be "relatively easy" and compelling to implement a Perl 5 compiler in Perl 6. So that'll be another plausible approach to using some CPAN modules.