I think, this is a more exact solution in perl: 
sub open_fd {
  my ($fd, $flags)=@_;
  use Fcntl qw/O_APPEND O_RDONLY O_WRONLY O_RDWR O_ACCMODE/;
  use POSIX ();
  use IO::Handle ();

  if( ($flags & O_ACCMODE) == O_RDONLY ) {
    $flags='<';
  } elsif( ($flags & O_ACCMODE) == O_WRONLY ) {
    if( $flags & O_APPEND ) {
      $flags='>>';
    } else {
      $flags='>';
    }
  } elsif( ($flags & O_ACCMODE) == O_RDWR ) {
    if( $flags & O_APPEND ) {
      $flags='+>>';
    } else {
      $flags='+>';
    }
  } else {
    POSIX::close($fd);
    return undef;
  }

  return IO::Handle->new_from_fd($fd, $flags);
}

[download]
How about the equivalent XS?

In reply to Re^2: How to convert a file descriptor plus access mode bits into a perl filehandle? by tfoertsch
in thread How to convert a file descriptor plus access mode bits into a perl filehandle? by tfoertsch

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