Autoincrementing letters in Perl

I do not understand autoincrementing letters in Perl.

This example seems perfectly understandable:

$a = 'bz'; ++$a;
ca #output

b gets incremented to c. There is nothing left for z to go to, so it goes back to a (or at least this is how I see the process).

But then I come across statements like this:

$a = 'Zz'; ++$a;
AAa #output

and:

$a = '9z'; ++$a;
10 #output

Why doesn't incrementing Zz return Aa? And why doesn't incrementing 9z return 0z?

Thanks!