Assume that, up until you were six years old, you were told by your parents that Santa Clause was real.
Now assume that you are directed to the witness chair in a courtroom and that you are sworn and put under oath to tell the truth. Then assume you are asked this question: "When you were four years old, did your parents tell you Santa Clause was real?"
If you answer "no," you have committed perjury.
Legally speaking, perjury is (1) lying (2) knowingly (3) while under oath. [Edit: "corroboration" is also required; see comments below.]
To prove that you have committed perjury, the prosecution will submit evidence to a subsequent jury. The prosecutor will call your parents to the witness chair. Under oath, your father will stoically state that he told you that Santa Clause was real and your mother will tearfully admit the same. This is evidence of a knowing lie. Then the prosecutor will call some clerk of the court who will testify that she/he put you under oath and that he/she correctly transcribed your testimony. This is evidence that you were under oath when you made your statement.
If the jury believes you were under oath and believes the testimony of your parents, the jury will vote to convict you for perjury.
Note: it does not matter that what your parents told you is untrue. What matter is WHAT you were told and WHAT you then said under oath.
As this relates to Schultz and Curley, it does not matter what McQueary actually saw. In fact, for purposes of assessing the perjury charges against Schultz and Curley, assume if you want that McQueary completely fabricated the whole shower scene. It would not matter one whit. This is why Dranov's version of events was excluded and is not legally relevant.
What matters is what McQueary SAID he saw to Schultz and Curley (and then whether Schultz and Curley correctly relayed what McQueary SAID he saw when they testified before the Grand Jury).
From what I have read, imo, it is likely that Schultz and Curley will be convicted of perjury.
What they said McQueary said to them is at odds with what McQueary says he said to them. Ditto Paterno.
Oversimplifying: McQ & Paterno testified that they said: "McQ saw something sexual." Schultz and Curley testified to Grand Jury that McQ & Paterno said: "McQ saw horseplay."
The element of "under oath" is not in dispute. If the jury believes McQueary and Paterno, Schultz and Curley will be convicted.