Is not swearing falsely, in the event that harm such as imprisonment results, considered a yamin ghamus? If not, then what is an example of a necessity that permits it?

Al-Yamin al-Ghamus (the perjurious oath) is the false oath by which rights are usurped or by which immorality and treachery are intended. It is called "ghamūs" (immersing) because it immerses its taker in Hellfire or sin. A false oath taken out of necessity (such as avoiding prison) is not considered a "yamin ghamus", as necessity permits swearing falsely if telling the truth would lead to unbearable harm. In such cases, it is preferable to use equivocations (al-ma'arid) to avoid lying.