If this is possible--I've never done it, mostly because I've never seen a need for it--then success will resolve around using the bless() function exported from threads::shared rather than the built-in version.

Whilst it is possible to gain access to threads::shared::bless() from within your XS code, the simplest method to test whether it will actually work for you, would be to export the RV (RETVAL) from your XS new() (perhaps renamed _new()) un-blessed; and then add a wrapper method in the .pm file that calls the XS _new() and then applies the modified bless() (imported from threads::shared) at the Perl level before returning it.

That should get you past the "Invalid value for shared scalar" problem. I do not expect it to be a complete solution. You will almost certainly have to add a CLONE() method to the package. And you will need to deal with the problem of multiple DESTROY()s. But one step at a time.

If you module isn't proprietary, and doesn't have a raft of dependancies, and you were prepared to let me have an installable copy, it might make solving the knock on problems quicker.