Handling Files in a Subproceedure

We are adding application security (to hide some fields) to an application
of mine. I want to use a subproceedure to check for authorization. The
problem is that I have never really grasped the concept to how to properly
handle file opening and reading. I have always taken the easy way out and
just opened and closed the file every time the subprocedure was called. Is
there a sample somewhere on how to do this properly?