#!C:/Perl/bin/perl.exe -w  
  
$My_File="D:/My_File.txt";  
  
$Target= "ADD_STRING_HERE";  
  
open($My_File_FP,"<$My_File") || die("This file will not open!"); 
 
while(<$My_File_FP>)  
{  
	if (/$Target/)  
	{  
		print " found $Target on line $. \n";  
		close($My_File_FP);  
	}  
	else  
	{  
		open($My_File_FP,">>$My_File") || die("This file will not open!");		  
		print $My_File_FP "\n  ADD_STRING_HERE \n";  
  
	}  
}  
close($My_File_FP);

use strict; 
my $file_name = "test.txt"; 
my $string = "TEST_STRING"; 
my $flag = 1; 
# 
# file should be exist. you can use any other programs to do this. for example touch under Unix system 
# 
if (!(-e $file_name)) 
	{ 
	open(FF,">$file_name") or die "Cann't crate file $file_name : $!\n"; 
	close(FF); 
	} 
# 
# open file in read/write mode 
#	 
open(FF,"+<$file_name") or die "error $!\n"; 
while(<FF>) 
	{ 
	if (/^$string$/) 
		{ 
		print "String [$string] is exist in the file [$file_name]\n"; 
		$flag = 0; 
		last; 
		} 
	} 
print FF "$string\n" if ($flag); 
close(FF);