tie @log, 'Tie::File', $logfile or displayError("could not open logfile");

while ($some_condition) {
    #do some other stuff, and
    unshift @log, $info_to_be_logged;
}

# find out how long the logfile is, and truncate it
$length = @log;
$truncate = $length - 150;
if ($truncate > 0) {
   $#log -= $truncate;
}
	
untie @log;