fake 2D subscripts "is rw" problem

matrix_test.pl6

#!/usr/bin/env perl6

class Matrix4x4 is Array {
    method at_pos ($i) is rw {
        self.Array::[$_, $_+1, $_+2, $_+3] given $i * 4
    }

    method perl () {
        self.defined ??
            $?CLASS.perl ~ '.new(' ~ "\n" ~
                (^4).map({
                    self[$_].map({.perl}).join(', ')
                }).join(",\n").indent(4) ~
            "\n)"
        !!
            'Matrix4x4'
    }
}

my $m = Matrix4x4.new: |@(^16);

say 'Before:';
say $m.perl;

say "\nChanging value at [1][1] from $m[1][1] to 123";
$m[1][1] = 123;

say "\nAfter:";
say $m.perl; # shows no change