bless($x, undef) already has a defined meaning, and should not be changed.
Its behaviour, including the warnings, arises naturally from the usual
treatment of undef when a string is desired and from the behaviour of
bless on an empty string. There is no need for specific documentation
of this behaviour.
The appropriate way to invoke unblessing is as a subroutine supplied by
a module. As noted, this is already available. There is no need for
the core to supply an unblessing facility.
This ticket should be closed.