use Fcntl qw( :flock );

open (LOG, "+< /path/to/log.dat") or die $!;
flock(LOG, LOCK_EX);
my @logfile = <LOG>;

unshift @logfile,
        "$date - $requesturi - ($httpuseragent)"
        . " $remorehost: $remoteport ($remoteaddr) -" 
        . " $httpreferer\n";

seek LOG, 0, 0;
print LOG @logfile[0..99] or die $!;
close(LOG) or die $!;