Perks as Spells is part of the Inventing New Spells section of GURPS Sorcery.
Weirdly it says that perks can't take limitations but then effectively lets them take them anyway:
- These perks cannot take either of the limitations above, because perks don’t take modifiers
- by making the perk into a sorcery spell, it is automatically bound by the Sorcery limitation
The "give the perk a little more power or versatility" suggestion is essentially saying to apply some kind of enhancement to offset the discount.
Like for example, if you had Fur as a perk, then to offset magical -10% you could take switchable +10%. This works out to 0% and in exchange for the benefit of being able to make the fur vanish, it MUST vanish in No Mana zones.