I guess the only reason is because the book may be a bit out of date. This book really focuses on how to use references. They probably created this case to serve as an example. In this chapter it is about using references with File Handles. I think that they were using this case as an example of how to create file handles on the fly with references.
I am not sure why this IO::File is used in this case. They introduce it only as a subclass of IO::Handle and give some examples on how to use it.
Thanks for the help guys,