Sorry, it's been 15+ years since I last used sendmail directly. :-)
In skimming the rfc, I thought that
The only syntactic units which requires preservation of case information are: text, qtext, ..., quoted-pair, ...
implied that it was case sensitive, because quoted-string quoted-string = <"> *(qtext/quoted-pair) <"> includes quoted-pair, and "cc" was defined with quotes, so I assumed it was quoted-string, and thus required case sensitivity. Plus every instance of `cc:` in the rfc was all lower-case. But if `"From"` is explicitly not in that exception, and `"From"` and `"cc"` are defined with similar syntax, I obviously mis-remembered how to interpret those.
And, after reading the rest of the replies, it's now readily apparent that the sendmail problem was really not specifying the other recipients where sendmail wants them, and the perl problem was not using a modern mail-sending module that are the root causes of the problem. :-) Thanks for the correction.