+101        $used{"FOO123Z"} = 1;
..snip..
  +323        my $new="FOO123Z";
  +324        my $len = length($new);
  +325        while ( defined $used{$new} && $len > 0 ){  ## Keep the first character the same
  +326          foreach my $c (("A" .. "Z"), 0..9){
  +327            $new =~ s/(.{$len}).(.*)/${1}${c}${2}/ if defined $used{$new};
  +328          }
  +329          $len-- if defined $used{$new} ;
  +330        }
  +331        $used{$new} = 1;