My vote is for share. I think there's always room for "pure perl" modules to sit aside their xs cousins. There are bound to be lots of people in a similar situation to you. I can think of 2 examples right now that are in CPAN:

• Crypt::Blowfish vs. Crpyt::Blowfish_PP
• Crypt::DES vs. Crpyt::DES_PP

If you do publish to CPAN, I think you should probably notify the author of the original module (just so he doesn't stubmle into yours and become surprised) and name your module similarly, but distinctively diffrent from its xs cousin (I think the _PP standard used by the 2 modules I refrenced above is good).