"A123","valueA1","valueB1"
"B234","valueA2","valueB2"
"C345","valueA3","valueB3" 
"D456","valueA4","valueB4"
"E567","valueA5","valueB5" 
"F678","valueA6","valueB6"

##</code><code>##

"C345","valueX1","valueY1"
"D456","valueX2","valueY2"

##</code><code>##

"A123","valueA1","valueB1"
"B234","valueA2","valueB2"
"C345","valueX1","valueB3" 
"D456","valueX2","valueB4"
"E567","valueA5","valueB5" 
"F678","valueA6","valueB6"

##</code><code>##

#!/usr/bin/perl
#use strict;
#use warnings;

open fh1,"<C:/Perl-Script/file1.txt";
open fh2,"<C:/Perl-Script/file2.txt";
open fh3,">>C:/Perl-Script/file3.txt";
@a=<fh1>; 
@b=<fh2>;

foreach $b(@b) {
	@k = split ' ', $b;
	$hash2{$k[0]} = $k[1];
}

foreach $a(@a) {
	@k = split ' ', $a;
	$hash1{$k[0]} = $k[1];
}

while(($k,$v)= each %hash1)
{$hash1{$k} = $hash2{$k} if (exists $hash2{$k})};

foreach $value (@a) {
	print fh1 $value;

}