context-singleton

contrive q (Animal)
  => class => Animal::
  => builder => q (new),
  ;

my $animal = deduce q (Animal);

# in tests (in new frame generally) you can override your singleton
subtest ... => sub {
  frame {
    # now we can inject different singleton
    proclaim q (Animal) => Test::Fixture::Animal->new;

    ...
  };
  # and q (Animal) is back
};

# Test::YAFT::subtest creates frame for you
Test::YAFT::subtest ... => sub {
  proclaim ...;
};