Formal:
* The treaty was broken by an act of perfidy, as the nation secretly planned to attack while claiming peaceful intentions.
* The betrayal of his comrades was a shocking act of perfidy that forever stained his reputation.
* The spy's perfidy allowed the enemy to gain a significant tactical advantage.
Informal:
* Their sudden change of plans after promising to help was a clear act of perfidy.
* He couldn't believe his friend's perfidy – betraying him for such a small gain.
* The company's perfidy in misleading investors about their financial status led to a major scandal.
Literary:
* The once loyal knight's perfidy shocked the kingdom, as he had always been seen as a paragon of virtue.
* The playwright used the character's perfidy to explore themes of betrayal and the fragility of trust.
Remember: "Perfidy" is a strong word, suggesting a serious breach of trust. Use it carefully to convey the gravity of the situation.