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  DB<1> %a = (a => 1, b =>2, c => 3); %b = (b =>2, c => 1, d => 4)
  DB<2> $seen{$_}++ for (keys(%a), keys(%b))
  DB<3> x grep { $seen{$_} > 1 } keys(%seen)
0  'c'
1  'b'

  DB<1> use List::Util qw{first}
  DB<2> %a = (a => 1, b =>2, c => 3); %b = (b =>2, c => 1, d => 4)
  DB<3> x first { ++$seen{$_} > 1 } (keys(%a), keys(%b))
0  'c'