my $lines_to_skip = 0;
LINE: while (<MSOUT>) {

    if (0 < $lines_to_skip) {
        $lines_to_skip --;
        next LINE;
    }

    ...

    if ($line =~ /^(Position|Spot|Mark) has rolled into$/)) {
        $lines_to_skip = 1;
        next LINE;
    }
}