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.

yeah man

select

https://perldoc.perl.org/functions/select#select-RBITS,WBITS,EBITS,TIMEOUT even links

http://man.he.net/man2/select

select links https://en.m.wikipedia.org/wiki/Select_(Unix) links https://pubs.opengroup.org/onlinepubs/9699919799/functions/select.html

EBITS is for error filehandles.

No. What does that even mean? It's used to specify the handles to monitor for exceptional conditions. See my answer for details.

It's used to specify the handles to monitor for exceptional conditions.

If you need it, you'd know it.

Perl's select is a thin wrapper for the system call of the same name.

From my select(2) man page:

The file descriptors in exceptfds will be watched for exceptional conditions. (For examples of some exceptional conditions, see the discussion of POLLPRI in poll(2).)

From my poll(2) man page:

POLLPRI

There is some exceptional condition on the file descriptor. Possibilities include:

• There is out-of-band data on a TCP socket (see tcp(7)).
• A pseudoterminal master in packet mode has seen a state change on the slave (see ioctl_tty(2)).
• A cgroup.events file has been modified (see cgroups(7)).

By the way, IO::Select presents a much better interface.