And, since I don't seem to be able to let this go, still shorter and sweeter:

sub standard_commas {
    return join q{, },
        @_ < 2 ? @_
               : (@_[0 .. $#_ - 2], qq{$_[-2] and $_[-1]})
    }

sub oxford_commas {
    return join q{, },
        @_ < 3 ? goto &standard_commas
               : (@_[0 .. $#_ - 1], qq{and $_[-1]})
    }
[download]

Tested. The output, which I won't inflict upon you again, is the same.