" Perjury is tough becuase it required proving "knowing lie" and the lie has to be "material."
Tell that to Scooter Libby.
I know the Libby case pretty well. Libbied misled the investigators, on purpose, to keep heat off the WH. The reson Libbies misleading was material was that the investigators were looking for sources of leaks (there can be more than one).
Contrast with Comey ADMITTING he leaked. I don't care if he lies about the reason, he's already confessed the deed.