sub ret_backtrace {
  ...

  my %i = caller_info($i);
  $mess = "$err at $i{file} line $i{line}$tid_msg\n";

  while (my %i = caller_info(++$i)) {
      $mess .= "\t$i{sub_name} called at $i{file} line $i{line}$tid_msg\n";
  }
  
  return $mess;
}