There is no one answer to this question since there is no one definition of perfect grammar. However, here are a few things that might be included in a definition of perfect grammar:
-All sentences are free of errors in grammar, punctuation, and spelling.
-Sentences are well-constructed and use correct grammar, punctuation, and spelling.
-Sentences are clear and concise.
-Sentences are easy to read and understand.
Of course, perfect grammar is subjective and what one person considers to be perfect grammar may not be what another person considers to be perfect grammar. However, the above listed items are generally considered to be hallmark characteristics of perfect grammar.