The problem is that in the first example your Content is a reference to a array with all your content values.
But in the second example your content is a reference to a array with only _ONE_ entry. anoteher way is to put your content data into a array or hash. Here is a example with a hash:

my %data = (
    ettablenr => '2',
    etcell2x1 => 'Task',
    etcell2x2 => '1',
    etcell2x3 => '2',
    etcell2x4 => '3',
    etcell2x5 => 'High',
    etcell2x6 => 'CSS',
    etcell2x7 => 'CCCC',
    etcell2x8 => 'Twiki Updation',
    etcell3x1 => 'Task',
    etcell3x2 => '4',
    etcell3x3 => '5',
    etcell3x4 => '6',
    etcell3x5 => 'High',
    etcell3x6 => 'DSS',
    etcell3x7 => 'DDDD',
    etcell3x8 => 'Twiki CSV',
    etrows    => '3',
    etsave    => 'Save table'
);

# you change something with $data{etrows} = 4; 

my $res = $ua->request(POST $url,
  Content_Type=>'form-data',
  Content =>[ %data ]
);
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