Perl 5 (or, more accurately, CPAN) compatibility mode is only really interesting in two cases: when converting a project piecemeal from Perl 5 to Perl 6, and in the time between when Perl 6 gets generally usable and when it gets sufficient library support with native Perl 6 code.

All of this depends on Perl 6 being generally usable and having a working Perl 5 compatibility mode.