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

my $outs = <<'EOS';
A B C
A B D
A D E
EOS

print $outs;

my %hash;
foreach (split ' ',$outs)
{
   $hash{$_}++;
}

print "Values occurring exactly twice:\n";
foreach (keys %hash)
{
   print "$_\n" if ($hash{$_} ==2);
}

__END__
A B C
A B D
A D E
Values occurring exactly twice:
B
D