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]

Three notes.

• Using IO::Handle is a step backwards. There's definitely no equivalent to IO::Handle in XS. You'd have to call the Perl method from C if that's what you wanted to use it. Fortunately, new_from_fd just ends up calling open, so there's no need to involve IO::Handle.

• POSIX::close($fd); is really misplaced. It doesn't belong there at all.

• In fact, it works out better if you convert the mode from the O_ constants to a Perl mode in a separate function.

So here's what we have in C-ish Perl:

use Fcntl qw( O_APPEND O_RDONLY O_WRONLY O_RDWR O_ACCMODE );

sub file_mode_from_c {
   my ($c_mode) = @_;
   $c_mode &= O_ACCMODE | O_APPEND;

   my $p_mode;
   if    ( $c_mode == O_RDONLY            ) { $p_mode = '<';   }
   elsif ( $c_mode == O_WRONLY            ) { $p_mode = '>';   }
   elsif ( $c_mode == (O_WRONLY|O_APPEND) ) { $p_mode = '>>';  }
   elsif ( $c_mode == O_RDWR              ) { $p_mode = '+>';  }
   elsif ( $c_mode == (O_RDWR|O_APPEND)   ) { $p_mode = '+>>'; }

   return $p_mode;
}

my $p_mode = file_mode_from_c($c_mode)
   or die("...");

open(my $fh, "$p_mode=", $fd)
   or die("...: $!");
[download]

So again, all you need to know is how to call open. I don't know the XS equivalent of open though. (Upd: Looks like PerlIO *PerlIO_fdopen(int fd, const char *mode);, thanks to Anonymous Monk's link to perlapio )