Since $! isn't set inside the file-to-be-evaluated, it must have been set by Perl while processing the file.

Yes, and completely irrelevant. $! cannot be used to determine if an error occurred.

Imagine that we are doing a file which (legally) returns undef, which might be reasonable if the code in the file is executed only for its side effects

No, it's not reasonable. The "do" module should return true on success just like "require" modules.

In reply to Re^3: Why do we need a \n with do FILENAME? by ikegami
in thread Why do we need a \n with do FILENAME? by rovf

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