#!/usr/bin/perl

use strict;

my %hash_1 = ( 'a' => 'b', 'c' => 'd', 'e' => 'f' );
my %hash_2 = ( 'a' => 'J', 'e' => 'K' );

my %new_hash;

$new_hash{$_}  = $hash_2{$_} ? $hash_2{$_} : $hash_1{$_}
  for keys %hash_1;

print map { $_ => " => " . $new_hash{$_} . "\n" }
  keys %new_hash;