TMTOWDI - I'd be tempted to use a glob like you did but with a regex. I don't think you need to really use -e
@files=reverse sort @files;
#should work because you don't have image.10.jpg
#if you do have image.10.jpg, then you will need to use
#image.09.jpg for example
next unless m/^$pre\.(\d)\.$post$/; #just to be sure
rename_and_log($_, "$pre." . ($1 >= 8 ? 9 : $1+1) . ".$post\n");
Sorry it's not very readable, but it should work. I think.
Update: I like merlyn's solution better.
the blue haired monk