perl 'advancing' variable facility..

Hi:
I don't understand the way perl 'advances' the variables along the range a-z, A-Z, and 0-9.
For example from a book

$a = "A9"; print ++$a, "\n";

gives B0 as expected,

but,
$a = "Zz"; print ++$a, "\n";

gives AAa.

Why?? why not Aa only!

also $a = "9z"; print ++$a, "\n";

gives 10.
and why not 0a?

Thanks in advance
V

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