#!/usr/bin/perl -w
use strict;

my %set1=( k1 => 1, k2 => 2, k4 => 2);
my %set2=( k1 => 1, k3 => 2, k4 => 2);

@set2{keys %set1}=undef;

while( my( $k, $v)= each %set2)
  { $v='undef' unless( defined $v);
    print "$k => $v\n"; 
  }