Global variables can seem like a handy way to pass state information around in a program, but the use of global variables only works well in very small programs. As the code base grows, you'll need to understand every subprocedure's impact on the global state in order to understand how the program works. This is a task that quickly becomes impossible.
To control the situation, only the main procedure should be allowed access to global variables; it can then pass that state information to subprocedures as parameters.
