Bhagavad Gita, Chapter 15, Verse 17

But there is another, the Supreme Purusha, called the Highest Self, the immutable Lord, who pervading the three worlds, sustains them.

A real, third category is introduced here, beyond the two already named, quietly supporting both.

Krishna doesn't stop at perishable and imperishable, he names something beyond both, present in all three worlds as a real, unchanging support. The framework keeps expanding to hold what it initially left out.