Hello! Thank you for your request.
I had thought of doing that for ages, but then I reworked the automatic re-connection logic, and decided that the change wasn't actually needed or useful.
https://github.com/dakkar/net-stomp/commit/2fa634de42ae305b7188b97a87907b83a0c66d93 this change is the minimal needed code (ignoring the important case when multiple writes are needed to send a frame).
Keeping in mind that ->send_frame will try quite hard to write the entire frame (using multiple calls to syswrite, or reconnecting if the socket goes away), what is the actual use case for returning what is, essentially, length($frame->as_string)?