Isn't another solution to make perks not variable, and clearly outlined in the contract as part of the employment package? You only have to worry about what your employees will do if you take away a perk when it's actually something you can consider taking away. If you treat it as part of the guaranteed compensation, you'll budget for it accordingly just like your employees do.

If you want to give bonuses, like the parent said make it a flat guaranteed amount, or tie it to a number based on company performance that people can see and make plans for (profit sharing with published numbers at regular intervals, preferably more often than it's paid out).