just before your main processing instructions 
foreach $key(%my_hash) {
  delete $my_hash{"$key"} if(!exists($my_hash{"$key"}));
}

[download]
that will get rid of empty keys--assuming you're not interested in these--and henceforth you can now iterate over keys of the hash.
the hardest line to type correctly is: stty erase ^H

In reply to Re^3: Printing only the "keys that have a value" in hash by aquarium
in thread Printing only the "keys that have a value" in hash by iphone

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