use Errno qw( EAGAIN EINTR );

my $rv = sysread($sock, $buf, $BLKSIZ, length($buf));
if (!defined($rv)) {
    if ($! == EAGAIN) {
        # Only reached you are using a non-blocking socket.
        # No data available at the moment. Try again later.
    } elsif ($! == EINTR) {
        # Only reached if you have a signal handler.
        # Interrupted by signal. Try again.
    } else {
        # Error!
    }
}
elsif (!$rv) {
    # EOF. Socket was closed
else {
    # Data was read
}