Perhaps the best thing to do is to remember that in "magic" string incrementation, digit/lowercase/uppercase characters increment from right to left, wrap within themselves, and generate a "carry" into the next left column (or a new left column character if it's already the leftmost (update: and see LanX's reply concerning the "type" of a new column)) when they wrap:

increment left col ("carry") <- '0' -> '1' -> .. '9' (digits)
                                 ^                |
                                 |                |
                                 +----------------+

increment left col ("carry") <- 'a' -> 'b' -> .. 'z' (lower case)
                                 ^                |
                                 |                |
                                 +----------------+

increment left col ("carry") <- 'A' -> 'B' -> .. 'Z' (upper case)
                                 ^                |
                                 |                |
                                 +----------------+
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String incrementation is pretty stable. I've never known it to change and I doubt it ever will (to preserve backward compatibility), so the best thing I can suggest is to experiment. Whatever results you get are the way "magic" string incrementation works.

c:\@Work\Perl\monks>perl -e "my $s = 'zZ9';  print qq{'$s' -> };  ++$s
+;  print qq{'$s'};"
'zZ9' -> 'aaA0'
c:\@Work\Perl\monks>perl -e "my $s = 'Zz9';  print qq{'$s' -> };  ++$s
+;  print qq{'$s'};"
'Zz9' -> 'AAa0'
c:\@Work\Perl\monks>perl -e "my $s = 'Zzzz9';  print qq{'$s' -> };  ++
+$s;  print qq{'$s'};"
'Zzzz9' -> 'AAaaa0'
c:\@Work\Perl\monks>perl -e "my $s = 'zzzz9';  print qq{'$s' -> };  ++
+$s;  print qq{'$s'};"
'zzzz9' -> 'aaaaa0'
c:\@Work\Perl\monks>perl -e "my $s = 'ZZZZ9';  print qq{'$s' -> };  ++
+$s;  print qq{'$s'};"
'ZZZZ9' -> 'AAAAA0'
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