If I were doing this, only the PV would be shared and I'd expect SvREADONLY to work until the scalar is destroyed.

I think Perl really needs to support this type of thing. I run into lots of cases where it would be nice to have Perl offer (efficient) read-only access to a block of memory that it didn't allocate and be smart enough to not try to free that block when the scalar is destroyed (and possibly call a hook instead). There are convoluted and inefficient ways to get Perl to kind-of do this.