Here's another way you could do it, which yields 4 arrays at the end, the union of the hash keys, the keys found only in hash1, those found only in hash2, and the intersection (found in both hashes).

I commented it fairly thoroughly in the hopes that it would help you during the initial stages of the Perl-learning process.

use strict;
use warnings;

# Create two test hashes, with some matching keys, and some keys only 
+in
# the first or second hash.
#
my %hash1 = ( 'a' => 1, 'b' => 2, 'd' => 3, 'f' => 4, 'h' => 5 );
my %hash2 = ( 'a' => 1, 'c' => 2, 'd' => 3, 'e' => 5, 'g' => 6 );

# Get the union of the two hashes, and save it in an array (@union)
my %union = map { $_ => 1 } (keys %hash1, keys %hash2);
my @union = keys %union;

# Declare arrays for values only in %hash1, values only in %hash2, and
# values found in both (the intesection).
##
my (@hash1, @hash2, @inter);

# For each item in the intersection, construct a flag which identifies
# which array to save it in.
#
foreach (@union) {
    my $flag = (exists $hash1{$_}? 1: 0) + (exists $hash2{$_}? 2: 0);
    (1 == $flag) and push @hash1, $_;
    (2 == $flag) and push @hash2, $_;
    (3 == $flag) and push @inter, $_;
}

# Display the results
print "[Results]\n";
printf "Union ........... %s\n", join(',', sort @union);
printf "Intersection .... %s\n", join(',', sort @inter);
printf "Only in hash1 ... %s\n", join(',', sort @hash1);
printf "Only in hash2 ... %s\n", join(',', sort @hash2);


__DATA__

[Results]
Union ........... a,b,c,d,e,f,g,h
Intersection .... a,d
Only in hash1 ... b,f,h
Only in hash2 ... c,e,g
[download]