No pecan pie cannot be left out overnight because when you leave the pecan pie out overnight at room temperature there could be bacteria that can grow easily and very fast on the pie and when you eat that pie it can make you sick.
The bacteria that grows on the pecan pie is not usually able to be seen unless you have a microscope but bacteria does eventually grow on pies.
You could eat the pie after leaving it out overnight but you do so at your own risk and it's not worth risking getting sick from it.
If you're not gonna eat the pie or serve the pecan pie within 2 hours you should refrigerate it and it will keep for 4 to 7 days before it starts going bad.