We've debated adding ,=
to Perl 6, but so far have decided it would too easy to confuse with the .=
operator visually. I suppose we could add ,=
just for completeness and then come back in six months and deprecate anyone who is using it. :-)
As for unifying with the .= operator, that wouldn't work at all in Perl 6 because we've taken over dot for method calls, and .= already indicates that you want to turn a non-mutating method call into a mutating method call. The append operator in Perl 6 is the ~=, but in Perl 6 the ~ character has very strong implications of stringification, and we wouldn't want to break that.
All that being said, it'd be trivial for you to add your own ,= operator to Perl 6, and that's probably where we should leave it.