I'm sorry, but you are *not* asking "If I write will it succeed". -w doesn't mean that. -w will also return true on filesystems that are full, or are out of inodes. All -w means is "does this resource have a write bit set for the current user".
Yes, perl is portable. That doesn't mean that you don't have to care about portability. For example, to make your code work on both Windows and Unix you have to care about whether lines of text end in \n or \r\n and to be rather careful about how you open files. Well, dealing with automounters is apparently another of those things where you need to be careful about portability.