open FH, '>c:\temp\tempfile';
the extra comma and greater-than sign help too.
The main point still stands - hard coding filenames through your code is not a good idea. I don't know whether it is such a great idea to "Always, always, always, use '/' as a directory separator" or not. I feel it's fine to use whichever directory separator is appropriate for your filing system, just don't hard-code it.
I'm not checking for errors or the like, because I was pitching the article at perl programmers who are competent, but who may not have programmed on more than one platform. As such I would be expecting them to be doing all sorts of neat stuff, but perhaps overlooking basic portability issues because they've never needed to address them before.
I didn't believe in evil until I dated it.