EBITS is for error filehandles. The select builtin is a thin layer over the underlying select(2) system call so familiarity with that can help (see your OS' manual pages or a tome like e.g. Stevens, Advanced Programming in the UNIX Environment).

Unless you've got a burning desire to program that close to the metal as it were you may be better served using IO::Select (or stepping even further back and using an event loop like Mojo::IOLoop, AnyEvent, or POE).

Edit: And I just bothered to pull my copy off the shelf and skim and I'm a bit vague still. That's filehandles which you want to know if/when they have errors or exception conditions present.