I'm also a bit confused about what you want. I think one of the other replies may hold your solution, but have you forgotten the Perl "magic" string incrementer:

c:\@Work\Perl\monks>perl -wMstrict -le
"for my $s (qw(Y Z AA  ZY ZZ AAA  ZZY ZZZ AAAA)) {
   my $t = $s;
   printf qq{'$t' -> };
   $t++;
   print qq{'$t'};
   }
"
'Y' -> 'Z'
'Z' -> 'AA'
'AA' -> 'AB'
'ZY' -> 'ZZ'
'ZZ' -> 'AAA'
'AAA' -> 'AAB'
'ZZY' -> 'ZZZ'
'ZZZ' -> 'AAAA'
'AAAA' -> 'AAAB'
[download]

If you already have string 'ABC', you can always go to the next step.