You should write automated tests such as unit tests, integration tests, system tests, that will run automatically on the code and yield an answer if all of them are successful, or if any of them fail.
There are many good practices for automated testing such as having daily builds or reaching a 100% test coverage. Here are some resources to get you started:
The Perl Quality Assurance project, also see their wiki.
Note that having automated tests is not a substitute for having dedicated software testers (and vice versa). By all means, they are both necessary for any serious operation.