Well considering Clarks superenhanced senses the possibility that he could be suprised is rather small. (In fact those episodes where he is it's really corny. All his friends throws a suprise birthday party in his house, yet he fails to notice them, for someone who can hear voices on the other end of town this is rather funny.) He could probably identify Perry by smell or the sounds of his steps.
Some fics has the notion that Clark can "soften" his body whatever that means. My take is that he rolls with the impact of a force, like a boxer. He has conditioned his body to respond to things like a human reflexivly. When Lois slaps him in the Tempus Fugit she doesn't break her hand either, despite that it should have been a complete surprise to Clark.
Another thing I always thought was rather funny is in DLW when Clark tries to listen and explains to Lois that he can't hear anything when she is babbeling in his ear and that it feels like a knife in his head. Hmm... he can swallow bombs with a smile but the missus voice brings real pain.
