Actually all attempts to write a Perl 6 compiler in Perl 5 have shown that it's not a very good idea.
If you want to cover only a small part of Perl 6, the one that's close to Perl 5, your chances are very good, and it runs reasonably fast. Maybe a few Perl5ism leak through, but all in all that's fine. (For example v6 works on that level).
However if you plan to write a full Perl 6 compiler on top of Perl 5, you can reuse less of Perl 5's power for Perl 6, and end up using Perl 5 as an assembly language. Perl 5 is a great high level language, but a rather poor assembly language, so your compiler and the run-time engine becomes slow, very slow.
That happened to the KP6 ("KindaPerl6") compiler, and I guess it will happen to all other Perl 6 compiler projects which use Perl 5 too heavily under the hood.