The perk gives 5% more damage reduction. According to the description of arcane shield it is supposed to increase the damage reduction of shield from 45 to 50 and on charged from 55 to 60.
In reality it gives 5% additional stacking damage reduction, like arcane charity would.
So either the description, or the type of damage reduction the perk gives is wrongy because 55% to 60% is a lot better than 55% and 5% stacking