That adds the -TP to the compiler commands for every file, rather than just the one that needs it. In this case, that doesn't seem to prevent the rest from compiling ok, but is that always true?

But I don't see how to use that syntax to remove unneeded options?

Didn't you see all the noise and errors that I'm seeing at the linker stage?

Using response files instead of huge command lines is a step forward over EU::MM--but automatically deleting them so the programmer cannot look to see what options were used and perhaps try adjusting them, before sitting down to try and work out how to pursuade build.pl to produce what you've discovered works, is three steps backward.