use v6;

  sub outer(*@vals) {
    my &helper = -> @prev, @rest {
      if @rest {
        # We've still got a rest to interate over.
        # We add all items of @rest[0] to the new @prev
        # in our sub-helper, and use @rest[1...] as new
        # rest, i.e. all elements of @rest except the
        # first one.
        helper [ *@prev, $_ ], @rest[1...] for @rest[0];
      } else {
        # We don't have to recurse further, so we
        # simply "return" @prev.
        take @prev;
      }
    };

    # @prev: Empty array
    # @rest: @vals
    gather { helper [], @vals };
  }

  my @a = outer [1,2,3],[4,5,6],[7,8,9];
  say join "\n", @a;
  # 1 4 7
  # 1 4 8
  # [...]
  # 3 6 8
  # 3 6 9