You said that need to install/compile Perl-5.8.0 to have threads, and want to share the modules with the Perl without threads. So, probably you have Perl-5.6.1 (or less) and and want make it work with Perl-5.8.0. What you are missing is that binarys of Perl-5.6.1 doesn't work with binarys of Perl-5.8.0! So, you can't share any module of Perl-5.6.1 with Perl-5.8.0, specially with XS.

** Note that threads are only stable at Perl 5.8.0+. Don't compile old versions to use it.

Graciliano M. P.
"The creativity is the expression of the liberty".