OK, let's walk through this.

A hash reference is just a reference to a hash, getting at the hash is a simple matter -- so we can ignore the "reference" part of your question for now, and focus on how to determine if two hashes share any keys. Let's start with two simple hashes:

%a = ( one=>1, two=>2, three=> 3 );
%b = ( two=>'two', three=>'three', four=>'four');
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We know the keys they have in common are two and three, but how to find that out in code?

First, let's rephrase the question a little bit: if I know all the keys in one hash, how do I find out if any of those keys exist in another hash?

Let's work backwards from that. How do we find out if the %a hash contains a key -- let's say, the key one? That would be exists:

if (exists $a{one}) { print "\%a has 'one' for a key" }
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Ok, so how do we find out what keys a hash has? How about keys:

print join(",", keys %a); # prints "one,two,three"
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So, if we get the list of keys for hash %a, all we have to do is loop through them and see if %b has any that match. Here's the long way:

my @common_keys;
foreach my $key (keys %a) {
   if (exists $b{$key}) {
      print "\%a and \%b have key '$key' in common\n";
      push @common_keys, $key;
   }
}
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Make sense? Of course, this type of activity -- looking through a list to find items that meet a criterion -- is so common that Perl has a function for it: grep. So here's a short version:

my @common_keys = grep { exists $b{$_} } keys(%a);
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Now let's add "references" back to the mix. Let's make references to our hashes:

$x = \%a;
$y = \%b;
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Now, we do as above, but dereferencing as appropriate:

my @common_keys = grep { exists $y->{$_} } keys( %{ $x } );
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We pass the whole hash referenced by $x to keys; and we use the dereference operator -> to make sure that exists looks at the hash referenced by $y.