Testing with databases can be tricky. if you are developing a system married to a single database then you can make some assumptions about your environment and ask the user to provide relevant connection information. but if you need to test a framework that uses dbi, particularly a framework that uses different types of persistence schemes, then it may be more useful to simply verify what the framework is trying to do -- ensure the right sql is generated and that the correct parameters are bound. dbd::mock makes it easy to just modify your configuration (presumably held outside your code) and just use it instead of dbd::foo (like dbd::pg or dbd::mysql) in your framework.