If you want to validate ResultsSet, then I guess it can be something like that

$result_or_error =  run_plan('facts', targets => $targets,
    _catch_errors => true,
  )
  $r = case $result_or_error {
    ResultSet: { $result_or_error }
    Error['bolt/run-failure']: { $result_or_error.details['result_set'] }
    default: { fail_plan($result_or_error) }
  }