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Data: Hashes (Associative Arrays)
  How do I process an entire hash?
...
  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?

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