Grease and flour a 9x5x3-inch loaf pan. Preheat oven to 350°.
Toss chopped pears with lemon juice in a small bowl; set aside.
In another bowl, combine streusel ingredients, except pecans, with a fork until crumbly, or pulse with food processor. Stir in pecans, if using.
In a mixing bowl, combine flour, sugar, baking powder and salt; stir to blend.
In another bowl, whisk eggs and stir in milk, vanilla, and butter.
Fold the egg mixture into the flour mixture and combine just until the dry ingredients are moistened.
Spoon half of the batter into prepared loaf pan; spread out to cover the bottom.
Sprinkle batter with half of the pears and half of the streusel.
Spoon the remaining batter evenly over streusel, spreading carefully to cover.
Sprinkle with half of the remaining streusel, the remaining chopped pears and then the remaining streusel.
Bake for 1 hour 15 minutes, or until a wooden pick inserted into center of cake comes out clean. Cool in pan on a wire rack for 10 minutes. Run a knife carefully around sides; invert the cake onto plate. Turn the cake, streusel side up, onto a rack and let cool on a rack completely. Enjoy.