has a chapter on subroutines; it's chapter 8 and it answers all your questions.
The error you're getting is caused by trying to read from a filehandle that was opened for output only. You can't do that. Although it's possible to open a filehandle that you can both write to and read from, that's an advanced topic and not suited for beginners.
Also, don't write to the file that you're still reading. Strange things will happen. Instead, write to a new file and rename that when you're done and sure that the old file may be removed. This also avoids some race conditions, and incomplete files if the program should crash before it's done writing.