"Looks like our plan went perfectly," he said drily, staring at the broken pieces on the floor.

drily 是形容词 dry（干燥的）加上副词后缀 -ly 构成的副词形式。dry 本义指"干燥"，引申为"不带感情的、冷淡的"。drily 在这里表示"用冷淡、讽刺的语气说话"，中文常译为"干巴巴地"。