have on 是穿着的意思,表状态,而put on 是披上衣服,表动作
The poet who has a black suit on is from Britain.
那个穿黑色衣服的诗人来自英国.
如果用put on 应该是：
The poet who is puttinga black suit on is from Britain.
那个正在把黑色衣服披到身上的诗人来自英国.