The problem is that when you do return (\@cols, \@hw);, you are returning references to the arrays, not the arrays themselves As it turns out though, that's what you want to do, lest those two arrays get mushed together. In your "main" code, just change

my (@cols, @hw) = new Itiv::Hw->all_hw($dbh, $filter);
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to

my ($cols, $hw) = new Itiv::Hw->all_hw($dbh, $filter);
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and treat $cols and $hw as array references.