To see what structure the first loop creates, use 
use Data::Dumper; print Dumper(\@input);

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You'll get the following output: 

$VAR1 = [
          [
            'a',
            '23',
            '11'
          ],
          [
            'b',
            '24',
            '15'
          ],
          [
            'c',
            '16',
            '19'
          ],
          [
            'd',
            '8',
            '2'
          ],
          [
            'e',
            '40',
            '41'
          ]
        ];

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The map line 

my @output = map [ $_->[0] ], @input;

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creates the output, it maps each element of @input to a reference to a new array containing its first element, i.e. 

$VAR1 = [
          [
            'a'
          ],
          [
            'b'
          ],
          [
            'c'
          ],
          [
            'd'
          ],
          [
            'e'
          ]
        ];

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The last cited line sorts the array indices (that represent input line numbers) by $c-th element (reversed), and than sets $c-th element of the output to be the order number. Thus, the first iteration processes the second column: 

$VAR1 = [
          [
            'a',
            3
          ],
          [
            'b',
            2
          ],
          [
            'c',
            4
          ],
          [
            'd',
            5
          ],
          [
            'e',
            1
          ]
        ];

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and the second one the next one: 

$VAR1 = [
          [
            'a',
            3,
            4
          ],
          [
            'b',
            2,
            3
          ],
          [
            'c',
            4,
            2
          ],
          [
            'd',
            5,
            5
          ],
          [
            'e',
            1,
            1
          ]
        ];

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($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

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In reply to Re^3: manipulating a data table by choroba
in thread manipulating a data table by v15

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