I really think such a move would be overly confusing
* Perl 7 coming out before Perl 6
* Perl 7 being compatible with 5
* Perl 6 being compatible with neither 5 nor 7
* Perl 7 coming out before Perl 6
* Perl 7 being compatible with 5
* Perl 6 being compatible with neither 5 nor 7
Explaining it is also a solution to the 'problem' they are trying to solve here, namely that 5.x seems 'dead' and 6.x seems 'alive'. A little explanation that the 5.x branch will continue to exist seems a lot easier (and less confusing).
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