select generally doesn't work properly if you mix in buffered reads and writes; select will perpetually think something is readable, or nothing is readable, because the buffering has over-read or under-read on the handle.

This means that whenever you're using select, either directly or indirectly through IO::Select or some event loop, then you must use sysread and syswrite (not print, <$fh>, read(), etc.).

You often end up doing your own buffering with sysread and syswrite, because often you're still dealing with line-oriented protocols. This is fairly boilerplate stuff, and it's recommended to use an event loop that does most of it for you (e.g. POE, Event, IO::Async, etc.).