Claws are an enemy in Braid. They appear and disappear out of green pipes at regular time intervals. If Tim touches them, he "dies" (see Time Manipulation).
This is a reference to the Mario games where Claw-like enemies called Piranha Plants were common in many areas of the games. However unlike Braid, in Mario, green pipes were often a shortcut to higher levels or secret locations.
However, in the Braid modification Silverbraid, in level 2-3 Hidden World, one does use the green tube to access a seperate area.