my  $struct = {...};  # the hash of hashes in the question
my  $found;
Data::Walk::walk \&wanted, $struct;
print join(" ",sort keys %$found) ,"\n";

sub wanted{
   if (/^values$/){
      foreach ( keys %{$Data::Walk::container->{$_}} ){
         $found->{$_}=1;
      }
   }
}