I presume this is your first module installation because you should be getting that error installing any module. Its not really an error in the sense of a bug. The problem is just as it says: "Permission denied". You don't have permission to make the directory /usr/local/man.

If you don't understand Unix permissions and how to fix that you should read these tutorials because you're going to run into this problem a lot. Its not unique to Carp::Assert or installing Perl modules.
http://www.osxfaq.com/Tutorials/LearningCenter/UnixTutorials/ManagingPermissions/index.ws
http://www.osxfaq.com/Tutorials/LearningCenter/AdvancedUnix/ugp/index.ws
http://www.osxfaq.com/Tutorials/LearningCenter/AdvancedUnix/ugp2/index.ws

In particular
http://www.osxfaq.com/Tutorials/LearningCenter/AdvancedUnix/ugp2/index.ws#eugid