I said it was a clue, not the whole solution :p

And you obviously missed the clue. Let me explain....

You want a sequence that goes A,B,....X,Y,Z,AA,AB, etc.... right?

Then all you need to do is use the auto-increment operator (++). Suggest you have a read of perlop (look for the "Auto-increment and Auto-decrement" section).

Cheers,
Darren :)