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

##</code><code>##

use Data::Dump;

my %a = ( a => 1, b => 2, c => 3 );
my %b = ( b => 2, c => 1, d => 4 );

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

##</code><code>##

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