Whats with the autoincrementation of a string

bingalee has asked for the wisdom of the Perl Monks concerning the following question:

Im trying to figure this out but I dont get it. Here's the code

#!/usr/bin/perl
use warnings;
$a = "A9"; print ++$a, "\n";
$a = "bz"; print ++$a, "\n";
$a = "Zz"; print ++$a, "\n";
$a = "z9"; print ++$a, "\n";
$a = "9z"; print ++$a, "\n";
[download]

OUTPUT

>perl auto2.plx
B0
ca
AAa
aa0
10
>
[download]

I get the first two outputs, A turns to B and 9 to 0.

But how does "Zz" become "AAa", "z9" become "aa0" and so on.. *confused*

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