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use List::Util qw( sum uniq );

...
     sum( map  $_->{$k}, @_ ) }
    uniq( map keys(%$_), @_ )
}

my %h1 = map { ( $_ => 1 ) } qw( a b c d e f g h );
my %h2 = map { ( $_ => 1 ) } qw(   b c d e f g h i  );
...
use Data::Dumper;
$Data::Dumper::Sortkeys=1;
print Dumper \%summed;