"that" is a determiner; it modifies "perfect sentence". Since no perfect sentence has previously been mentioned in the text, "that" in this case refers to the concept of a "perfect sentence" that we are supposed to already understand from experience or common knowledge.
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"that" is a determiner; it modifies "perfect sentence". Since no perfect sentence has previously been mentioned in the text, "that" in this case refers to the concept of a "perfect sentence" that we are supposed to already understand from experience or common knowledge.