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DB<2> $seen{$_}++ for (keys(%a), keys(%b))
DB<3> x grep { $seen{$_} > 1 } keys(%seen)

use Data::Dump;

...

my %seen;
pp grep { $seen{$_}++ } keys %a, keys %b;

pp grep { state %seen; $seen{$_}++ } keys %a, keys %b;