A settlement release usually covers all past and current claims known and unknown, and perjury wouldn't change that.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
If the settlement agreement was procured by fraud, it can possibly be rescinded.
If the defendant settled in reliance on plaintiff's false representations, the truth of which was concealed, that would arguably constitute fraud.
If the defendant can prove the plaintiff's testimony was perjurious, the settlement could possibly be rescinded on the basis of fraud.
With $1+ million at stake, a rescission action might make sense.
Moreover, giving false testimony under oath is perjury, which is a felony.
If the individuals who know the plaintiff perjured himself are willing to come forward, the plaintiff could possibly be charged with perjury, although prosecutions for perjury in civil cases are extremely rare.
I would strongly urge you to consult immediately with a litigator skilled in this area of the law.
PLEASE NOTE THAT THIS RESPONSE SHOULD NOT BE CONSTRUED AS A LEGAL OPINION OR ADVISE, AND DOES NOT CREATE AN ATTORNEY-CLIENT RELATIONSHIP. IN ORDER TO RENDER A LEGAL OPINION OR ADVISE, THE RESPONDING ATTORNEY WOULD NEED FAR MORE INFORMATION THAN HAS BEEN PROVIDED, AND WOULD NEED TO BE RETAINED PURSUANT TO A WRITTEN FEE AGREEMENT.