Since there was no further discussion, I've applied the patch as
96b697999ffc92903f9d0748fba8d281aeed6067
and am closing this ticket
--
Karl Williamson
---
via perlbug: queue: perl5 status: open
https://rt.perl.org/Ticket/Display.html?id=134234
[perl #134234] CODESET doesn't behave as documented
by Karl Williamson via RT