while (1) {
    my $res = sysread ($fh, my $data, $cfg->{max_line_length});
    if (not defined $res) {
        die "Input reading failed: $!\n";
    }
    elsif ($res == 0) {
        sysseek ($fh, 0, SEEK_CUR);
        sleep (0.5);
        next;
    }      
    <do stuff with $data here>
}