$o = {
    "a": {
      "x": [1,2,3],
      "y": [4,5,6],
    },
    "b": {
      "s": [7,8,9],
      "t": [9,5,1],
    },
  };
  # Expect "(error)" in the output data since strings are not map-indexable
  $z = $o[[1]]["x"];