# reading file into array and then process it 
# and write out to the same file:

	my $file	= "./test.txt";

	open (LOG, "+<$file");
	flock LOG, 2;
	my @logfile=<LOG>;
	seek (LOG, 0, 0);
	truncate (LOG,0);
	foreach my $line(@logfile){
		# do some manipulations with $line...
		print LOG $line;
	}
	close LOG;

##</code><code>##

# trying to optimize performance and memory usage 
# by replacing foreach() with while()

	my $file     = "./test.txt";
	my $tempfile = "./tempfile".(rand()*9999);

	open (LOG, $file);
	flock LOG, 1;

	open (TMP, ">$tempfile");
	flock TMP, 2;

	while (my $line = <LOG>){
		# here we do some manipulations with $line
		print TMP $line;
	}
	close TMP;
	rename($tempfile,"$file");
	close LOG;