As scorpio17 mentioned, if your modifications are enhancements, then the best thing to do is submit a patch to the author and get them incorporated in the next release. If you have the time to do so, and are planning on submitting a lot of patches, as to become a co-maintainer of the module. There are a number of free code collaboration sites that can help with this (http://github.com is a popular one).
in reply to Modifying a CPAN Module and Uploading as a New One?
In regards to legality, if you made your changes during work hours, you may want to check with your boss that your company doesn't have some sort intellectual property rule that says anything you write during business hours is the property of the company. In which case the changes belong to them, not you (technically) and aren't yours to distribute.
Fortunately, if you're using open source software at your company, odds are your employers are savvy to the economics of the open source world and would recognize the benefits of allowing you to contribute your changes back to the community. If not, try to educate them :)
If in the event that you're not able to get your changes into cpan; consider setting up a company wide minicpan and adding your modules to it.