Perjury requires testifying falsely under oath. Mistaken testimony is not enough, the testimony has to be knowingly false. Take the Barry Bonds case, for example. If Barry knew he had used steroids and testified under oath that he had not, then he committed perjury. But, if Barry testified that his trainer gave him "the cream" and "the clear", and that Barry did not know or understand that these contained steroids, then (if true) he would not have committed perjury by denying that he had knowingly used steroids.
The mere fact that each side in a case has a different version of the story to tell does not mean that one side is lying or committing perjury. Ultimately, the judge or jury will have to decide between the two versions, and determine which one, if either of them, is true.
Perjury is intentionally lying under oath. To be clear, being "wrong" is not necessarily the same as commiting perjury. If the testimony in question goes to the heart of the lawsuit, and further assuming you are correct in stating that it has been demonstrated in court that the plaintiff's testimony is wrong, then I imagine that the plaintiff will lose.
Please note that I am a New York attorney and cannot advise you as to the application of Washington state law. For that reason, you may wish to consult a local attorney.
Good luck to you.