# I'll use psuedo-code since you did :^)
while (my $line=<>) {
	print LOG $line;
	if ( ++$linenumber == 90% of max ) {
		LOG == LOG1 ? open LOG2 : open LOG1
	}
	if ( $linenumber == max ) {
		LOG == LOG1 ? LOG = LOG2 : LOG = LOG1
		LOG == LOG1 ? close LOG2 : close LOG1
		LOG == LOG1 ? rename(LOG2) : rename(LOG1)
		$linenumber = 0
	}
}