Test::Builder is what most Test modules are written with these days. It lets them quietly coordinate with each other and frees the authors from having to worry about the details. It was written in 2001 and in that decade there's been an explosion of testing modules.
A decade later, Test::Builder is starting to show its age and limitations. Its assumptions and biases are restraining the Perl testing community. Perl has moved on, too. When Test::Builder was written, testing was still a "new" thing. Now its a given. We have a real object system now and a sophisticated community to take advantage.
Enter Test::Builder2. A total rewrite of Test::Builder to remove its biases and let test authors do whatever they can dream up while still being the solid iron core of Perl testing and remaining compatible with Test::Builder. It takes advantage of things like Mouse (that's a small Moose), method wrappers and roles. Counter-intuitively, it does less than Test::Builder does while providing more opportunities.
Schwern has a grant for Test::Builder2 from the Perl Foundation and if he doesn't release something by PPW he will be sleeping with the fishes.