As I'm sure you have figured out already, if you want to avoid autovivication in this sort of operation, you can either use Data::Diver (as suggested by tye), or you can rearrange your conditionals:

if ( exists( $$x{fred} )) {
   print "fred defined\n";
   print "dave defined\n" if ( exists( $$x{fred}{dave} ));
}
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Obviously, if you need to do this sort of thing a lot, with arbitrarily deep structures, using the module will make your coding a lot easier and cleaner.