A socket is just a stream. How do you know when one rain's water ends and another begins in a river?

You're going to have to write some kind of mark on the stream so the receiving end knows when things are done, or send and out-of-band message. One kind of mark is to simply shut down the socket on the sending end (close $to in Sendfile). The client can sense this. If you want to be absolutely sure the whole thing got over there, make the last few (fixed length) bytes a checksum of some kind.

If you insist on keeping the socket open, then you're going to have to send a marker just like you are now. Just make sure it's NEVER going to happen in mid-stream though. You could mimic what multipart MIME does and send a long, unique string at the beginning of the stream and another at the end. (Heck, doing that lets you set $/ to the closing string. Some nice code bumming if you're up for it.)