my $logfile = 'c:/test.txt';
my $tmpfile = 'c:/tmp.txt';
my $maxsize = 100;
my $size = -s $logfile;
my $need_to_dump = $size - $maxsize;
if ($need_to_dump > 0 ) {
    open LOG, $logfile or die $!;
    open TMP, ">$tmpfile" or die $!;
    seek LOG, $need_to_dump, 0;
    print TMP $_ while <LOG>;
    close LOG;
    close TMP;
    unlink $logfile or die $!;
    rename $tmpfile, $logfile or die $!;
    unlink $tmpfile or warn "Can't unlink $tmpfile $!\n";
}