How many "requisite CPAN modules" do you need in Perl6, and how complete does it have to be overall, before you can implement what you want in it? And how stable do you need it to be. If Perl6 (e.g., Pugs) is *close* to being able to do what you want, you could consider starting in Perl6 now, and helping to fill in the gaps.

But, as others have said, if Perl6 is nowhere near being able to do what you want now (e.g., you've got a list of eight CPAN modules you need, some of which have dozens of other CPAN modules as dependencies, or you really need the kind of stability and performance that only come after a year-long feature freeze), then use Perl5 now -- but try to write nice clean modular code so that individual parts of it can be migrated one at a time to Perl6 later, (leaving the remaining parts as Perl5 code, via Ponie or whatever).