This means that you can now write (for instance) with <nixpkgs> { ... }; in release.nix, rather than { nixpkgs } : with nixpkgs { ... };
The underscores are ugly and the .pl extension is an implementation detail that shouldn't be visible to the outside. Also, get rid of the *.in files. It's not really necessary to generate them. And I was always modifying the wrong file.