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

my %a = ('a' =>1, 'b'=>2, 'c'=>5, 'd'=>19);
my %b = ('b'=>12, 'd'=>32, 'z'=>77, 'y'=>5);

my @common_keys = 
   do{	
        local %_;
        $_{$_}++ for(keys %a, keys %b);
        delete @_{(map{ ($_{$_} == 1) ? $_ : ()}keys %_)};
        sort keys %_; 
   };

print "@common_keys";