#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

my %hash;

$hash{'A'.'B'} = 1;
# note a mistakenly written comma between 'C' and 'D'  
$hash{'C','D'} = 2;
$hash{'E','F'} = 3;

print Dumper(\%hash);
print exists $hash{'CD'} ? "YES\n" : "NO\n";