I think one issue here is that test authors need a way to know they are running on a cpan-tester box. I think if there were a clear way to signal various things, (like no interactive prompts, on a cpan-tester's box, no live db tests, etc) then module writers would take advantage of it.

There are already some mechanisms to help support this in Module::Build (notes, prompt, y_n) and ExtUtils::MakeMaker (PERL_MM_USE_DEFAULT, prompt). Both of them have systems that allow you to prompt the user, or switch to a default if running in a non-interactive mode.

If a module is using either of these systems and the Makefile/Build.PL is run with STDIN attached to something that doesn't look like a terminal then the user shouldn't be prompted for input.

As a tester, maybe you know of a flag or env var or something that would tell us we are on a cpan-tester enviornment and should behave accordingly?

I think that's the wrong distinction to be looking at. The whole point of cpan-testers is that it gives feedback on how well modules build. If you do something different on cpan-testers from what you do on a normal build some of the utility disappears.

Instead we need to look at whether the build is happening in an interactive manner or not, and have appropriate default actions when the session isn't interactive.