Write tests for your bugs before you fix the bugs. Check your test coverage with Devel::Cover before you refactor some code. If not all lines that are affected by your planned refactoring are covered, write tests. If you add new features, write tests before implementing the features. Don't waste time: don't write tests for mature code that you don't plan to change in the near future. It is no problem that you are late. The biggest problem is that your core API is probably not as clean as it should be for nice tests, simply because you did not need a clean API. Your test strategy depends on your existing code base. Maybe it is ok to refactor (modularize) your configuration code because it would not be too much work. But generally it is a bad idea to refactor mature code just to test it easier.