Data: Hashes (Associative Arrays)
  How do I process an entire hash?
  What happens if I add or remove keys from a hash while iterating over it?
  How do I look up a hash element by value?
  How can I know how many entries are in a hash?
  How do I sort a hash (optionally by value instead of key)?
  How can I always keep my hash sorted? 
  What's the difference between "delete" and "undef" with hashes?
  Why don't my tied hashes make the defined/exists distinction?
  How do I reset an each() operation part-way through?
  How can I get the unique keys from two hashes?
  How can I store a multidimensional array in a DBM file?
  How can I make my hash remember the order I put elements i into it?
  Why does passing a subroutine an undefined element in a hash create it?
  How can I make the Perl equivalent of a C structure/C++  class/hash or array of hashes or arrays?
  How can I use a reference as a hash key?

##</code><code>##

if(@{$refa}{keys %{$refb}}) {
    #
}