I am reading documentation on select command and I can't figure out, what these RBITS,WBITS,EBITS mean.

Well, I came to guess, that probably, RBITS - means Read Bits, WBITS - Write Bits, but what about EBITS, then?
What does this construct mean? What is it used for? Manual doesn't explain anything. Just throws these:

select RBITS,WBITS,EBITS,TIMEOUT
my ($nfound, $timeleft) = select(my $rout = $rin, my $wout = $win, my $eout = $ein, $timeout);

without any explanation.