Hi, the manual way of doing this is to stuff the files into a hash, and see which hash keys have a count greater than 1. Below is the basic outline of such a script, but you would need to expand the hashes to include which file lines came from and possibly their line numbers, which you can get with

my $line = __LINE__;
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then you would need to rewrite your File1 and File2. Some methods for that are shown in Search Replace String Not Working on text file. Either seek and truncate, or reopen the filehandle with >.

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

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

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

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