open(LOG, "+<$logfile") || die;
flock(LOG, LOCK_EX) || die;

while (<LOG>) {						
    if (1..10) {
        chomp;
        push @entries, $_;
    } else {
        last;
    }
}

pop @entries;
unshift @entries, $log_string

foreach $entry (@entries) {
    print LOG $entry."\n";
}

close LOG;