It's not so much the length, it's whether or not perl data structures, regular expressions, modules, etc., would make the program "better". I still write Korn shell scripts, and one I'm working on now is 140 lines long (it's mainly a wrapper for some external commands), and there's currently no reason to write it in perl, though it wouldn't be too hard to do if it became necessary.
If you do program in Korn shell, I recommend learning about its (regex-like) patterns, parameter expansions, command substitutions, compound commands, background processes, and co-processes (and whatever of this that applies if it's bash or some other shell). These things can push back the need for rewriting in perl.
