The phrase "perfect substitution" means that two things can be used interchangeably without any difference or preference. They are exactly the same and can be swapped without any impact or change. Full definition
Awesome — and those sound like perfect substitutions! (downshiftology.com)
Usually doing rice flour in place of a starch will make the dough way too wet because the starch takes up a lot more liquid but coconut flour is a super thirsty flour so I think you made the totally perfect substitutions! (recreatinghappiness.com)
Cauliflower once cooked for awhile purees into a smooth and creamy mixture and I thought it would be a perfect substitution for potatoes. (wheatfreemom.com)