I second merlyn's suggestion to use File::Basename for three reasons:

1. What if they decide to turn on -w (like they should)? They get a "Use of implicit split to @_ is deprecated" warning. That's no fun.
2. What if they move to a system that doesn't use \ as the directory separating character (like UNIX)? It could happen, and your code doesn't handle /'s. Of course, not chopping off the path would cause the config file to always be stored in the same directory as the script, instead of where ever the script happens to be called from, which might not be such a bad thing after all...
3. What if they change the name of the script to something that doesn't have a .pl extension (more common on *nix systems)? Your code clobbers the name of the script. For example, if they called their script whatever, your config file would become whate.cfg and I don't think that's what you mean.

merlyn's regexp/File::Basename solution handles all of these cases and it's quite a bit easier to understand. Plus it doesn't modify $0 which could be useful (or not, depending; you can always modify his to do so).

All in all, that certainly was an interesting way of using split and it took me a minute to figure out what you were doing. :-)

Cheers.

bbfu
Seasons don't fear The Reaper.
Nor do the wind, the sun, and the rain.
We can be like they are.