..... that a witness made a material false statement under oath.
Suborning perjury is even more difficult to prove and would equally require strong objective proof (e.g. a taped conversation instructing a witness to lie). But, if proven, it is a serious offense as it is a frontal assault upon our fundamental belief in the rule of law and the integrity of the tribunal or court listening to sworn testimony.
Imagine a Senator instructing a nominee or a witness to lie during Supreme Court confirmation hearing?
Imagine a prosecutor instructing a witness to lie in a murder trial?
Imagine a sitting President instructing his former lawyer to lie under oath during a Congressional investigation about Russian interference in a US election when that President was fully aware that a foreign adversary colluded with his campaign?
No small thing.