What makes you so sure it won't work on Windows?

The :raw IO layer is equivalent to the binmode and I see no unixisms in your code that would prevent the program from working on Windows.

I just saw - you're opening the same file twice. This won't work under Windows - you'll have to seek back and forth in the one file to copy the data from the end to the front.

Update2: I tested your code on Win32 and it just works.