Whoever, in a trial, hearing, investigation, deposition, certification or declaration, in which the making or subscribing of a statement is required or authorized by law, makes or subscribes a statement under oath, affirmation or other legally binding assertion that the statement is true, when in fact the witness or declarant does not believe that the statement is true or knows that it is not true or intends thereby to avoid or obstruct the ascertainment of the truth, is guilty of perjury. It shall be a defense to the charge of perjury as defined in this section that the statement is true.