Fake library
Have a private implementation of the used library and make sure this one gets loaded instead of the real class. (e.g. by tweaking @INC and loading the module early.
examples/fake-module/t/lib/WWW/Mechanize.pm
package WWW::Mechanize; use strict; use warnings; our $VERSION = 'fake'; sub new { return bless {}, shift; } sub get { ... } 1;