Originally Posted by waltonalice

Dunster probably diagrammed sentences as a kid. The clause, "whoever won't waste it," is the object of the preposition, but, within the clause, "who" is the doer of the action/subject of the verb, so you use "who [subject form]" rather than "whom [object form of pronoun]." When you see the diagram of the sentence, it's easier to see the purpose that the pronoun is performing and to choose the correct version of the pronoun.