The main thing make_immutable does is generate an optimized constructor for your class. You have to do this manually, because Moose doesn't know when you are done defining the class. Rather than going through all of the class metadata during construction, the optimized constructor is built to handle the specific attributes defined in your class.

namespace::autoclean and no Moose; are doing similar things. They remove imported functions in your class so that they can't be used as methods. This includes things like extends and has. But namespace::autoclean will automatically clean all of the imported functions, so it is much easier to use. Rather than no X; no Y;, a single use namespace::autoclean; will take care of all of the imports. It will also clean functions from modules that don't provide an unimport method to remove functions.