The line;

push @rows, @fields;
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Needs to be;

push @rows, [ @fields ];
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Here is an example;

#!/usr/bin/env perl

use strict;
use warnings;
use Data::Dumper;

my @fields = qw/ fred one two three /;
my @out;
push @out,  [ @fields ] for ( 1..3 );
print Dumper \@out;
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Which produces the output;

$VAR1 = [
          [
            'fred',
            'one',
            'two',
            'three'
          ],
          [
            'fred',
            'one',
            'two',
            'three'
          ],
          [
            'fred',
            'one',
            'two',
            'three'
          ]
        ];
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