Since you mention various shells and BAT files in the same sentence I'll throw this out there. If you need to run on Windows and UNIX, perl (or ruby, or python) may be the only way to go. While it can be tough to make certain it remains portable, there's not much hope in writing anything cross platform in any shell language.

The other comment I'd include is if any modules are required. If any/many CPAN modules are required to write it in perl, it may be easier to use shell.

If a valid goal is learning, why not write it in both. If you are familar with shell scripts and learning perl write what you know and learn by porting.

Finally, what usually carries the day - in which can you do it faster! Right or wrong, that usually decides the issue for me.