WASHINGTON — Agriculture Secretary Sonny Perdue on Thursday defended a proposal that would replace a portion of food stamp benefits with pre-assembled boxes of shelf-stable goods delivered to recipients’ doorsteps — an idea one lawmaker called “a cruel joke.” The idea was first floated last week in the Trump administration’s 2019 budget proposal, tucked inside a larger plan to slash the Supplemental Nutrition Assistance Program, or SNAP, by roughly $213 billion — or 30 percent — over the next 10 years. The backlash was fierce and immediate, from nutrition experts, advocacy organizations and lawmakers alike.