I found this among the perk entries, "Mod Secondary Value Weight". Anyone's got a clue what it does, or if it's just some unused leftover from GECK or something?

There are magic effects that use "Dual Value Modifiers" e.g. lightning damage effects that damage 1.0 x health and 0.5 x magicka
Perhaps that was meant to be used on such effects to make them deal equal damage to both values?