I assume you're referring to the GPL here, and you're wrong. Read section 1 and 2 of the GPL, you may use and modify any code however you see fit, provided you keep the license intact, make changes clearly visible and retain the copyright notice. Taking bits of GPL software into your codebase is such a modification. This does not mean your codebase suddenly becomes or needs to become GPL'ed itself, only the parts of the program that were originally under the GPL stay under the GPL. If you distribute your derivative work (i.e. the codebase into which you've integrated the GPL'ed code, or parts thereof, depending on the integration * ) it does need to be licensed under the GPL, but you are right, this does not need to happen if you use the work internally only.

As an aside, the situation is a bit interesting for independent contractors who write code for other companies and uses GPL'ed software as the base, because this does constitute a distribution of derivative work and as such their work needs to be released under the GPL.

* Update: I should probably clarify this before someone jumps on me. If you decide to use a GPL Perl module in your proprietary content management system and want to distribute said CMS, this does not mean you need to license the whole CMS under the GPL, unless the CMS can only work with the GPLed software and not with an alternative module (i.e. the module is at the heart of the CMS functionality and essential to it). However, if you take a piece of a GPL module and put that into your own module you are more than likely creating a derivative work, and as such your module needs to be licensed under the GPL. Feel free to correct me on this if you feel I have misunderstood the GPL, but this is how I believe it pertains to Perl modules/programs.