in reply to The correct way to return arrays from a module

You return two references to arrays. _Not_ two arrays. 
my ($cols_ref, $hw_ref) = new Itiv::Hw->all_hw($dbh, $filter);

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should work. 
for my $col ( @$cols_ref ) {
  print "$col\n";
}

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Boris

In Section Seekers of Perl Wisdom