One of the beauty parts of the perjury trap is that a corroborating witness is not required to obtain a conviction. All it takes is one witness against the defendant, and supportive circumstantial evidence.
If Michael were to be tried for the murder of the Five Families heads, a prosecutor would have to find at least one corroborating witness to even bring the case to trial. But a US Attorney could put Michael on trial for perjury simply because he denied under oath being responsible for the massacre, and Frankie's sworn testimony contradicted him. "Circumstantial evidence" could be as simple as convincing the jury that he benefited from the demise of the other Dons. In effect, it'd one man's word against another, and it'd up to a jury to determine who's telling the truth. Even if they refused to believe Frankie and acquitted Michael, he'd still have had his "legitimate" cover torn to ribbons. Thus the Senate committee couldn't lose: If Michael had been tried and convicted of perjury, he could go away for five years on each count without prosecutors having to try him for any of the crimes he lied about. If he were acquitted, he'd have still been dragged through a long, arduous and supremely ugly trial that would have exposed a mass of allegations against him. The Nevada Gaming Commission then would be under pressure to yank his licenses, at minimum.