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use strict;
use warnings;
...

    undef %hash;
}

 push @{ $hash{ $header[$_] } }, $columns[$_] for 1 .. $#columns;
      ^            ^                 ^            ^    ^
...
      |            |                 + - Column value
      |            + - Column heading
      + - Generate a HoA, where: keys are column names; values are ref
+erences to lists of column entries