unless (exists $cache{$inner})
{
  $cache{$inner} = 0;
  $cache{$inner} += $_->{value} for @{ $foo->{$inner} }{@keys};

  # This might be a little faster, because perl doesn't have to build the temporary list @{ $foo->{$inner} }{@keys}
  # $cache{$inner} += $foo->{$inner}{$_}{value} for @keys;  
}
$sum = $cache{$inner};