For instance, say you have a module Foo, and some other module Bar uses Foo. If Foo were to change its API in a non-backwards-compatible way, this would cause Bar to break until it is updated to use the new API. Foo can't just depend on the fixed version of Bar, because this will cause a circular dependency (because Bar is already depending on Foo), and this doesn't express intent properly anyway - Foo doesn't use Bar at all. The ideal solution would be for there to be a way to specify conflicting versions of modules in a way that would let CPAN clients update conflicting modules automatically after an existing module is upgraded, but until that happens, this module will allow users to do this manually.