Perfect Contrition would be sorrow based on love. One feels sorrow for the sin because it hurts God, and we do not wish to hurt God.
Imperfect Contrition would be sorrow or partial sorrow based on anything else (ie fear, coersion, etc.)
No sin is forgiven by God unless there is true sorrow for it (even imperfect sorrow, such as fear of divine punishment) and a firm resolution not to commit it again.
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