My solution, designed to take any number of hashes, not just two. Encapsulated in a function named add_hashes. Uses some functions in List::Util.

use List::Util qw( sum uniq );

# pass list of hashrefs.
# returns a hash (list).
sub add_hashes(@)
{
    map { my $k = $_; $k =>
     sum( map  $_->{$k}, @_ ) }
    uniq( map keys(%$_), @_ )
}
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Test it:

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  );
my %h3 = map { ( $_ => 1 ) } qw(     c d e f g h i j );
my %h4 = map { ( $_ => 1 ) } qw(       d e f g h i j k );

my %summed = add_hashes(
    \%h1,
    \%h2,
    \%h3,
    \%h4,
);

use Data::Dumper;
$Data::Dumper::Sortkeys=1;
print Dumper \%summed;
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