sub someFunc {
  my $args = [
    { client_number => 1, MM => 2, DD => 3, S => 4, },
  ];
  for (keys %$args) {
    my $idx = $args->[0]{$_} - 1;
    if (defined $_[$idx]) { $args->{$_} = $_[$idx], next }
    die "$_ not defined";
  }
  # hooray
}