The morphology of oriented lamellar stacks in roll-cast triblock copolymer films exhibits a nonuniform distribution of defects. The lamellae predominantly orient parallel to the plane defined by the velocity and velocity-gradient directions of the flow field between the counterrotating cylinders. However, the orientation effect along the flow direction in as-cast films is much stronger. Annealing at elevated temperature results in uniaxial symmetry about the main orientation axis of the lamellar normals. The response of a triblock copolymer structure, composed of alternating glassy and rubbery lamellae, to deformation along the lamellar normal direction is evaluated by comparing as-east and annealed films. The defect distribution governs the macroscopic and microstructural behavior. Annealed samples deform uniformly without necking, and lamellar tilting and folding into the "chevron" morphology occurs about any axis perpendicular to the stretching direction with equal probability. In as-cast samples, a unique "one-dimensional" necking is observed macroscopically. The cross section is reduced only along one spatial dimension (the velocity gradient direction of the flow field), and lamellar tilt is observed only about a single axis (the velocity direction of the flow field). These observations highlight the dominant role of defects in perpendicular deformation of block copolymer structures composed of alternating glassy and rubbery layers.