package A;
...
package A::a;
use base 'A';
...
package A::b;
use base 'A';
...
package B;
use A; # A is abstract and contains 'factory' methods, so we
       # will actually use instances of A::a or A::b
...
package C;
use B;
...
package D;
use C;
use XYZ; # not our code...something we installed
...