By casting evil glances a person's eyesight becomes weak. This is because by guarding one's gaze, one is fulfilling gratitude for the blessing of sight and the reward of gratefulness is the increase of blessings as it is mentioned in the Qur'aan Shareef:

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"If you are grateful then We will most definitely increase you in blessings."

(Sura Ibrahim; Ayat 7, Juz 13)

            Casting evil glances on the other hand is ingratitude and the punishment of ingratitude is severe, as is mentioned in the Qur’aan Shareef:

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"And if you are ungrateful then know that My punishment is severe."