I've read about it in Perl Best Practices book and I've tried it. It looks very promising (especially with Readonly::XS helper module).

But Devel::DProf show it's still use tie()ed scalar (and so call FETCH() on each scalar access) even with Readonly::XS installed.

This was sadly news, because I feel like with Readonly::XS it should just 'mark' SV as read-only and doesn't tie() it. :-(