y is a function which is explained in perlop. What it does is substitutes for any character in one list, the corresponding charater on another list.

What I did was use ord and chr to figure out what the ASCII representation of 9 was, and what came right after that. Then I used tr to move the range A-Z so that they came right after the range 0-9, so that I could convert any digit into a number just by taking ord and subtracting off the ord of 0.