Since you mentioned that the output is what you want what you want your code to look like is:

#!/usr/bin/perl -w
use strict;
use warnings;
use diagnostics;

my @data = [
   '17-02-2005','18-02-2005','19-02-2005',
   '20-02-2005','21-02-2005','22-02-2005',
   '23-02-2005','24-02-2005'
];

my %labels = (
   'C' => [0,0,0,0,7,0,0,2],
   'A' => [0,0,0,0,0,0,5,4],
   'B' => [0,0,0,0,0,0,0,0]
);

foreach my $item (sort {$a cmp $b} keys %labels) {
   #       replaced
   #push @{$data[$item]}, $labels{$item};   
   #         with
   push @{$data[0]}, $labels{$item};
}

use Data::Dumper;
print Dumper @data;
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The problem is that in the push you are using $data[$item] in which case $item (being the hash keys 'A','B','C') not being a numeric are being converted to 0 (and hence pushing things onto the first array in your AoA (@data) and a warning is thrown. Which is why I replaced $item with zero in the code above.

-enlil