Be careful with that state. It won't do what (I think) you want.

use 5.010;
sub foo {
  my ($a, $b) = @_;
  sort grep { state %seen; $seen{$_}++ } keys %$a, keys %$b
}

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

say join ' ', foo(\%a, \%b);
say join ' ', foo(\%a, \%b);

__END__
b c
a b b c c d
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lodin