IDE :: Use An OnEnterFrame Event To Call A Function Just Once?

I need to use an onEnterFrame event to call a function just once. Here's the code:

my_object.onEnterFrame = function() {
if (this._y == 100) {
my_function();
}
};

So say i drag "my_object" to a y-coordinate of 100, how do i make it so it only calls the function once. Then if i drag the object away from y=100, and then back to y=100, i want it to call the function again.

I'm using the onEnterFrame event b/c i need it to constantly look for the object to be at y=100, maybe except when its actually at 100.