Why does a Pesi grow wise from punishment of a Letz?
Rashi: Due to seeing his afflictions, he repents.
R. Yonah: Refer to 19:25:1:2.
Malbim: Above, it says "Letz Takeh u'Fesi Yarim" (19:25). Here it discusses Chachmah, i.e. receiving the laws of Chachmah to conduct according to them. There, it says Yarim; if they hit the Letz, the Pesi will get Armah (cunning) to be careful. Via Onesh of a Letz - what he deserves via judges or bi'Ydei Shamayim - it is recognized that it is due to his sin. The Pesi will understand that there is reward and punishment, and he will accept the laws of Chachmah, for they are from Hashem.
What do we learn from "uv'Haskil l'Chacham Yikach Da'as"?
R. Yonah: When a Chacham grows wise and understands what he erred about in deed, he will guard the Da'as in his heart and not forget it. It is permanently carved on the tablet of his heart, for he stumbled and erred about it. One understands Divrei Torah only if he stumbled in them (Gitin 43a). It says about a Navon "v'Hochi'ach l'Navon Yavin Da'as" (19:25) - amidst your rebuke, he understands to be careful [also] about other things.
Malbim: A Chacham received the laws of Chachmah, but he does not know them as clearly as what is sensed (Da'as). Via punishment of the Letz, the Chacham will see how Hashem punishes Midah k'Neged Midah one who mocks the laws of Chachmah. Then he will acquire Da'as - knowledge as clear as what is sensed.
