<i>...module that relies on specific undefined behavior...</i><p>You call Perl 5&#x27;s use of reference counting &quot;undefined behavior&quot;?<p>It has been documented behavior for as long as Perl 5 has existed.  The resulting destruction mechanics have been relied on by the core module <a href="http:&#x2F;&#x2F;perldoc.perl.org&#x2F;SelectSaver.html" rel="nofollow">http:&#x2F;&#x2F;perldoc.perl.org&#x2F;SelectSaver.html</a> for the same time.  Furthermore any interesting XS program is likely to make use of the relevant macros SVREFCNT_int and SVREFCNT_dec.<p>Therefore this is NOT something that can be changed in a point release.  It is a fundamental part of how Perl 5 works.  And the fact that it is a fundamental incompatibility with Perl 6 plans is something that people have been aware of for a decade.