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use strict;
use Data::Dumper;
...
my %h3 = map {$_ => $h1{$_} + $h2{$_}} keys %h1;

print Dumper \%h3;

$VAR1 = {
          'c' => 3,
          'b' => 10,
          'a' => 7
        };

my %h3 = map {$_ => $h1{$_} + $h2{$_}} keys %h1;

my %h3 = map {$_ => $h1{$_} + $h2{$_}} keys %h1, keys %h2;