Either @non_dupe_rows is very poorly named, or my @non_dupe_rows = do { my %seen;grep !$seen{$_}++, @rows }; isn't doing what you think it's doing:

my @rows = qw/ 1 2 3 4 1 2 /; # 1 & 2 are dups
my @non_dupe_rows = do { my %seen;grep !$seen{$_}++, @rows };
print "@non_dupe_rows\n";
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prints

1 2 3 4
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If you really want non-dup rows, try

my @rows = qw/ 1 2 3 4 1 2 /; # 1 & 2 are dups
my @non_dupe_rows = do {
    my %seen; 
    $seen{$_}++ for @rows;
    grep $seen{$_} == 1, keys %seen
};
print "@non_dupe_rows\n";
[download]