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("...: $!");
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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 )