At some point in every Perl class I have taught, I have tell someone "fix your syntax problems before you try to diagnose your logic problems."

So, what happens with your code after you fix your errors?

As a side note I'd suggest you rethink your design using a hash for the reading-list books and then do a single pass through the catalog file, reporting all matches as you go. The logic will be cleaner and the places where you can get the wrong index go away. (I know you think that the multiple arrays solution is 'cleaner'; Code will prove you wrong. What will happen the first time that the reading-list or the catalog changes? (Nota Bene: You have already demonstrated that problem.) This has all the potential of being a full-time maintenance headache. Not that I am objecting, mind you, cleaning up code like this on contract is what I do for a living....)