The old code put them together is just because curl need to use the content of the list after setopt and during the curl perform, and the set opt and the perform is in different method of the class, so they are put as class member.
But the 2 usage of the slist is not same, one for header is non const, need be a class member var and the alias one is const, It not change anyway. local static is the best for it as I see, but if you insist, I can change it be the way like the non const, or split it to 2 step commits?