The open function normally takes three arguments, the filehande, an indicator which says if it is reading, writing, appending, or a pipe open, and the filename, (or command in the case of a pipe).

In your code snippet, you have used the old two argument form of open, without the indicator to say if it is reading or writing. If you don't specify, then perl will open for reading, which is OK in this example, but not good practice, as you could get confused in other situations.