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.