AIDS deniers use bogus copyright claims to censor critical Youtube videos

At what point does the frivolous (and dishonest and illegal) filing of DMCA takedowns cross the line into defamation itself? I think it would be libel rather than slander, but IANAL.

As far as perjury is concerned, is it even possible to institute a civil suit or force a criminal suit for it, or is that purely a matter of prosecutorial discretion?