I would think index() would be the fastest, though a benchmark is at hand. Check out perldoc -f index.

UPDATE:

I must say, I do find it irritating when people post first, are wrong and then modify their post to be an almost direct copy of another post. My suggestion was new, because the first reply by friedo had suggested using substr rather than index. After my post he had changed his. If you want to change your viewpoint/modify your stance on a subject, than at least state that you have UPDATE(d): your node.