*combos = foldr_c( \&outer_prod, [[]] );

    sub outer_prod {
        my ($xs, $ys) = @_;
        [ map do { my $x=$_; map [$x, @$_], @$ys }, @$xs ];
    }


    say combos( ['a','b'], [1,2,3] );
    # [['a',1],['a',2],['a',3],['b',1],['b',2],['b',3]]