No idea what is put there. I believe it's just scratch space for certain operation but what makes or uses those SVs I have no idea.

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Update
I went looking through my poke_my notes and refreshed my memory. My understanding is that the other stuff in the pad is indeed scratch space for opcodes.

From perlguts
A scratchpad keeps SVs which are lexicals for the current unit and are targets for opcodes. One can deduce that an SV lives on a scratchpad by looking on its flags: lexicals have SVs_PADMY set, and targets have SVs_PADTMP set.

-Lee

"To be civilized is to deny one's nature."