The Lesson: never be useful.
Do your time, shut your mouth, the Law ain’t got your back.
I think the real lesson here is: Don’t break the Law.
also, if the threat was indeed serious enough it seems reasonable that Millie could have spoken to someone before testifying under oath.
Or she could have asked for witness protection.
On the other hand, I can understand the need to be so restrictive — you want to put the criminals away no matter what threats they make. Frankly, there could easily be NO extenuating circumstances permitted for perjury, and in fact every reason to make it far stiffer: if you don’t testify, you’re basically a co-conspirator in every crime for which they are being tried, about which you could testify.
Yeah, and you could take all of their friends who didn’t know anything, and punish them too! Because which is more likely, that they really didn’t know their friend was a criminal, or that they’re just covering for them. That’ll teach them for being friends with a dirty criminal!