Windows has a very different file permission model to Unix and integrates them only minimally.
On Windows, all this function can do is to change the "read only" flag, which is turned on if $mode & 0200 does not pass.
i.e. it only checks if u+w is missing from the bitmask, and if it is, it sets the read only flag.
The executable flag cannot be set as Windows determines it based on file extension.
The write flag cannot be set as Windows determines write access based on ACLs, which are not integrated here.
