Rakudo Star is out, and so begins the next great wave of interest and use of Perl 6. The next several releases will improve performance, fix bugs, add features, port or create more libraries, and—in all likelihood—improve and otherwise clarify the Perl 6 specification.
The Perl ecosystem has room for other projects, however.
For example, one of the clearest benefits Perl 6 has over Perl 5 is its portability to other virtual machines and runtimes. By design Perl 6 encourages multiple implementations. Perl 5 is its own specification; in many places, what Perl 5 is is solely what Perl 5 happens to do. Sometimes that behavior gets enshrined in the specification tests, but other times it's folklore and institutional community knowledge.
Just as Parrot's Lorito project intends to make Parrot at least an order of magnitude faster, so too a reorganization of Perl 5 internals could make amazing things more possible.
What if there were a project to implement a minimal set of Perl 5 on the Parrot virtual machine as a prototype and exploration of how much of Perl 5 you can support, the effort it takes to do so, and what kind of utility you can expect? Parrot's compiler tools let the Rakudo developers write most of Perl 6 in Perl 6; surely it's possible to write Perl 5 in a similar fashion. (Credit to other projects such as Rubinius and PyPy for demonstrating that such things are possible.)
I know other projects have attempted this in the past. Perhaps the best place to steal information is Bradley Kuhn's masters thesis, Considerations on Porting Perl to the Java Virtual Machine.
As Jesse wrote in his comments, bug-for-bug compatibility isn't necessary. Nor is full compliance with the existing Perl 5 test suite. A simple proof of concept to produce the 80% of Perl 5 most people use in most programs should suffice. (Parrot gives you a lot of that anyway.)
As a bonus, you get cheap and easy interoperability with Perl 5, access to Parrot features such as multidispatch, grammars, continuations, and bytecode serialization, and you could even replace some of the uses of Perl 5 within Parrot's and perhaps even Rakudo's configuration and build processes.
It doesn't even have to be a pony of full size.
I've little more than a passing interest in most things Perl6ish; my general position is that it looks a fun an interesting research project, a melting pot of ideas, some of which we can steal in Perl5 (5.10's say, given/when, //, and so on). But one thing that would interest me a lot would be real VM-level support for continuations.
I did notice in the Rakudo Star announcement that nobody's implemented non-blocking IO yet. When that lands, suddenly a Perl6 implementation of IO::Async + CPS which isn't quite as hackish with lexical closures as continuations might start to look more plausible...
For Perl5 in perl5 then Language-P might be interesting? http://search.cpan.org/dist/Language-P/ & http://github.com/mbarbon/language-p