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use strict;
use warnings;
...
        my $amount = $articles[$j]->findvalue('./quantity/text()')//"0
+";
    print "$unitname;$article;$amount\n";
}

Entity A;A3236;554
Entity A;B9735;386
...
Entity B;D3938;
Entity B;E5722;
Entity B;F3596;

findvalue('./quantity/text()')//"0";