Without going to specific jurisdictions perjury is "the offence of willfully telling an untruth or making a misrepresentation under oath". It therefore requires an untruth intentionally made with the intent to deceive while under oath - not stated in this definition is that the deceit must be material to the matter being decided.
First, pleas are not given under oath and second, a not guilty plea simply means "the prosecution does not have enough evidence to convince the jury beyond reasonable doubt that I committed this specific crime I am accused of in the specific particulars alleged ... I think" which is hardly a position that can be a lie.
The fact that a jury or judge does not believe a witness is not evidence that that witness has committed perjury. The witness could believe an untrue thing and be faithfully reporting their untrue belief. The witness may miss-remember an event and be faithfully reporting that mistaken recollection. The witness could be telling God's own truth and simply be unable to convince the jury or judge of this - that is, they are wrong not to believe.
To prove perjury the state has to provide sufficient evidence that the witness knowing told a material lie with the intention of deceiving the court or tribunal. This is not easy.
There is an expectation that defendant's will use any and all methods in their defense. Even methods that are, strictly speaking, not legal. It is the prosecution's duty to prove guilt beyond reasonable doubt - the defendant has to prove nothing. If the prosecution's case is so weak that the defendant lying under oath is enough to derail it then it was too weak to take to trial!
In addition, prosecutors have a duty to the public to use limited resources (like their own and the court's time) judiciously. Is it really in the public interest to pursue a convicted murderer who has just been given a 12 year sentence on a charge of perjury for which the punishment will be 12 months served concurrently?