Roles are the Perl way to ensure a class adheres to a certain design. In Linux you can OPEN a socket or a file or a device driver or whatever else. They are treated the same because they conform to a certain interface. You might want plug in different sorting algorithms without knowing the details of how they work. A role is just one language's implementation of a concept with the goals of reusing code, minimizing dependency, and designing with flexibility in mind. Sub classes and abstract classes have greater dependency because changing a parent class changes every child class. Often times you'll want to change behavior of a child object, but.. oh wait all that code is in the parent.

HackaMol - OO Perl for molecular hacking PDF explaining design

Add Dry-Run testing to classes

http://blogs.perl.org/users/ovid/2010/01/roles-without-moose.html