What is the test for determining whether a witness has been found guilty of perverting the course of justice?

In making this determination, one looks to such factors as "the importance of the witness' testimony in the prosecution's case, whether the testimony was cumulative, the presence or absence of evidence corroborating or contradicting the testimony of the witness on material points, the extent of cross-examination otherwise permitted, and, of course, the overall strength of the prosecution's case." (Delaware v. Van Arsdall (1986) 475 U.S. at p. 684.)