#!/usr/bin/perl -w
use strict;
use Data::Dumper;

#build some test data here.
#create the lists of overlapping values
#and unique values
my @list_1 = (1 .. 20);
my @list_2 = (10 .. 40);

#define our sets
my (%hash1, %hash2);

#we are not interested in the values just
#the keys for this exercise so we use a
#hash slice, the @hash{@list_of_keys}
#construct to build our sample hashes.
@hash1{@list_1} = ();
@hash2{@list_2} = ();

#filter out our stuff.
#I like do blocks since they return stuff.
my @in1only = do{
    #I localize %_ here and set it to %hash1
    #you could also use my %temp_hash_for_this_scope 
    #for example but the effect is the same.
    local %_ = %hash1;

    #again we see the hash slice thingie this time
    #we use it with delete which it just so happens
    #can work with a hash slice. Here we use it to
    #get rid of keys from %hash2 that might be in
    #our %hash1.
    delete @_{keys %hash2};

    #finally we return a sorted list of the keys left
    #over from the delete call above. These are keys
    #unique to %hash1 only
    sort keys %_;
}; #<---- don't forget the semi-colon for the do block!!!!

#same as above... well kinda, just in reverse :)
my @in2only = do{
    local %_ = %hash2;
    delete @_{keys %hash1};
    sort keys %_;
};

print "in one only\n";
print Dumper(\@in1only);
print "\n";
print "in two only\n";
print Dumper(\@in2only);
print "\n";
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in one only
$VAR1 = [
          '1',
          '2',
          '3',
          '4',
          '5',
          '6',
          '7',
          '8',
          '9'
        ];

in two only
$VAR1 = [
          '21',
          '22',
          '23',
          '24',
          '25',
          '26',
          '27',
          '28',
          '29',
          '30',
          '31',
          '32',
          '33',
          '34',
          '35',
          '36',
          '37',
          '38',
          '39',
          '40'
        ];
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