_show_call_stack() if (!$retval);

[...]

sub _show_call_stack {
  my $max_depth = 7;
  my $i = 1;
  if ($log->is_debug()) {
    $log->debug("--- Begin stack trace ---");
    while ( (my @call = (caller($i++))) && ($i<$max_depth))    {
      $log->debug("$call[1] line $call[2] in function call[3]");
    }
    $log->debug("--- End stack trace ---");
  }
}