I don't think the Punisher could win this one. First off, there's absolutely no way he could know that L suspects Light. In the Death Note universe, L is the world's greatest detective and keeps all of his work completely secret. And Light kills criminals all around that world, not just in Japan, so I'm not sure that the Punisher would even be able to discover that Light was in Japan. And, if the Punisher did suspect Light and followed him, Ryuk may tell Light like he did with Raye. Ever way, I say Light is able to kill him, because Light is too much smarter.