ie. how would you store the selected column header's names into $rows, so that the following would work?

I am not sure if I got your question right, but assuming that $rows contains a reference to an Array of Hashes, where they keys are your columnnames like:

my $rows = [
            { 'column number 4' => 'data row 1, col 4',
              'column number 2' => 'data row 1, col 2',
              'column number 5' => 'data row 1, col 5',
            },
            { 'column number 4' => 'data row 2, col 4',
              'column number 5' => 'data row 2, col 5',
              'column number 2' => 'data row 2, col 2',
            }
        ];
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you could do

my %rownames;
$rownames{$_} = $_ for sort keys %{@{$rows}[0]};
unshift @$rows, \%rownames;
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to add an additional row at the top which contains the hash keys as values

[
          {
            'column number 2' => 'column number 2',
            'column number 5' => 'column number 5',
            'column number 4' => 'column number 4'
          },
# ...
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