sub consensus {
   my %counts;
   $counts{$_}++ for @_;

   my ($consensus) =
      sort { $counts{$b} <=> $counts{$a} }
      keys(%counts);

   return ( $counts{$consensus} / @_ >= 0.7
      ? $consensus
      : undef
   );
}
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Up to you to decide what to do when consensus returns undef because a consensus wasn't achieved.