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use warnings;
use strict;
...
for my $sku (sort keys %items) {
    $csv->print(select, [ $sku, map { $items{$sku}{$_} } @columns ]);
}

for my $product ($dom->findnodes('/products/product')) {
    my $sku = $product->findvalue('sku');
...
        $items{$sku}{$name} = $url;
    }
}