I expect most of the Perl 5 code reuse to happen by loading Perl 5 modules in Perl 6, not by using (semi)automatic translation.
A prototype for that already exists (search for blizkost), and it works for some frequently-used modules already (CGI.pm, Tk).
Parsing Perl 5 is fragile, and only works if you neglect certain things (like function prototypes). If you do that, you'll arrive add PPI (p5 code) or STD_P5 (p6 code).
Another thing to try is the MAD emitter that Larry hacked into perl5 core before the 5.10.0 release, which uses perl to parse Perl 5 code, and emits an XML-like description of the parse tree.
- links to (nearly) everything that is Perl 6.