If I say "I swear under penalty of perjury that the earth is flat" in a cease and desist letter, even if I know that the earth is not flat, I cannot be found guilty of perjury because it is not a context in which I can perjure myself - I am not under oath in a court of law, and there is no other law that constrains my freedom to say such a thing. Title 17, Section 512(c)(3)(A)(vi) says that a valid DMCA takedown request must contain an assertion, under penalty of perjury, that the author is authorised to act on behalf of the copyright holder, and as a result making that claim falsely would be perjury. But the reason it's perjury is that the law says that the claim is under penalty of perjury, not that someone wrote "Under penalty of perjury" at the beginning of the sentence.