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

open (FILE1, '<', 'File1.txt') or die "Unable to open file1.txt for reading : $!";
open (FILE2, '<', 'File2.txt') or die "Unable to open file2.txt for reading : $!";

my %lines;
while ( <FILE1> ) {
    chomp;
    $lines{$_}++
}
while ( <FILE2> ) {
    chomp;
    $lines{$_}++
}
open (FILE3, '>', 'File3.txt') or die "Unable to open file3.txt for writing : $!";

for ( keys %lines ) {
    next if $lines{$_} > 1;
    print FILE3 "$_\n";
}