use strict;
use warnings;
my %hash1 = (a => 1, b => 2);
my %hash2 = (a => 1, c => 3);
my %hash3 = (d => 1, c => 3);

# Method 1
# Create a hash for tallying the number of times keys occur in the other hashes.
my %key_bucket = ();
for my $key (keys %hash1, keys %hash2, keys %hash3) {
  $key_bucket{$key}++;
}
my @duplicates = grep {$key_bucket{$_} > 1} keys %key_bucket;

print "Method 1:\n";
print "$_\n" for @duplicates;


# Method 2
# Create a list of unique keys, then check how many hashes each key appears in
print "Method 2:\n";
my %unique = map {$_ => 1} keys %hash1, keys %hash2, keys %hash3;
for my $key (keys %unique) {
    my (@matches) = grep {exists $_->{$key}} \%hash1, \%hash2, \%hash3;
    print "$key\n" if @matches > 1;
}