I have pulled up no information on what $/ means.
You'll find that in perlvar
which is where all punctuation-named variables are documented. It is the input record separator. Its default value is newline, so it is often used instead of "\n" because it needs fewer key strokes. This might not work well if the input files do not have the same format as the ouput one (for exemple an input file with line ending on "\r") and $/ was not modified with proper care (read: with local
and a small enough scope).