sub _debug_fd {
    return fileno STDERR unless defined $cur_self;
    if ( _debugging && !defined $cur_self->{DEBUG_FD} ) {
        my $fd = select STDERR;
        $| = 1;
        select $fd;
        $cur_self->{DEBUG_FD} = POSIX::dup fileno STDERR;
        _debug("debugging fd is $cur_self->{DEBUG_FD}\n")
          if _debugging_details;
    }
    return fileno STDERR unless defined $cur_self->{DEBUG_FD};
    return $cur_self->{DEBUG_FD};
}