Are these pears that you have grown yourself or that you bought? What is happening here is a result of the pears being picked after they were fully mature. Essentially they were left on the tree a bit too long. Pears are ready to pick when you twist the fruit on the branch slightly and it snaps off on it's own. They may need to sit on a counter for a few days to soften, but they're ready at that stage. If left on the tree much past that stage, they'll start to discolor inside.