And with a very small change, your sum's too:

my %sum;
$csv->column_names ($csv->getline ($fh));
while (my $h = $csv->getline_hr ($fh)) {
    print "$h->{Name} => $h->{Cost}, $h->{Inventory}\n";

    no warnings "numeric";
    $sum{$_} += $h->{$_} for $csv->column_names;
    }

print <<EOS;
Total Inventory: $sum{Inventory}
Total Cost:      $sum{Cost}
EOS

=>

pickles => 2.99, 12
vegemite => 4.00, 8
nuclear sub => 22.50, 4
Total Inventory: 24
Total Cost:      29.49
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