use Tie::File;

$YOUR_LOG_LINE = "whatever you want.\n";

my @log;  
my $o = tie @log, 'Tie::File', 'log.dat' or die $!;

$o->flock;                       # Default is conveniently LOCK_EX

unshift @log, $YOUR_LOG_LINE;    # Add line to beginning. 
$#log = 99 unless @log < 100;    # Truncate to 100 lines if necessary.