If by "will Perl5 modules be compatible" you mean "is that the plan?" then yes, the plan is that you'll be able to use most perl 5 modules (including ones that use XS) via an embedded libperl on platforms where that works and you'll be able to use most pure perl 5 modules otherwise via the P6 project called "v5".
For a little more detail about these plans see the bit about "Mixing P6 and ..." in this post.