open my $fh, '<', $filename;

my $line            = '';
my $track           = 0;
my $max_line_length = 1024;    # or whatever

while (defined(my $char = getc $fh)) {

    $line .= $char;

    if (++$track == $max_line_length or $char eq "\n") {
        print $line;
        $line  = '';
        $track = 0;
    }
}

close $fh;