This is NOT a "Light leak" Isue.
It Is an electronic Ground Shielding Isue
Most people would not even notice this UNTILL you run CHDK with Long exposures
I have done alot to find the Exact reason
for this isue.
and i have found it.
if you use Black thick paper it will NOT fix the problem
i used tinfoil over the back of the CMOS about the size of the screen then poped the screen back in over it.
its a ground shield isue .
this is what mine WAS doing (Note the purple Haze) on the Left side.i had the SAME isue to but i dont now:)