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.