Besides, didn't you know passthrough is DEPRECATED?
Sorry to disappoint you, but you don't get to hang me on that nail. I explictly said further back I planned to use 'small' which uses the passthrough technique but is NOT deprecated. The one you are referring to is literally named "passthrough". I never have and never did have any intentions of using it. But if you read through the docs you will realize that it is deprecated because it is no longer needed thanks to CPAN's support for configure_requires, not because it is crap.
From the module docs (bold mine):
A small Makefile.PL will be created that passes all functionality through to the Build.PL script in the same directory. The user must already have Module::Build installed in order to use this, or else they'll get a module-not-found error.
- passthrough (DEPRECATED)
This is just like the small option above, but if Module::Build is not already installed on the user's system, the script will offer to use CPAN.pm to download it and install it before continuing with the build....
This option has been deprecated and may be removed in a future version of Module::Build. Modern CPAN.pm and CPANPLUS will recognize the configure_requires metadata property and install Module::Build before running Build.PL if Module::Build is listed and Module::Build now adds itself to configure_requires by default.