Your @data initially has one element, a reference to an array of date text. Is this what you want? I suspect not. I am guessing your code is only very close to what you want.

The "errors" are warnings. "A" and such evaluate to zero in a numeric context so your labels are pushed on to the first or zero-eth element of your data array. Since people don't usually want to convert "B" to 0, the warning is issued.

The $item index to $data in this line causes the warnings:
push @{$data[$item]}, $labels{$item};

Likely you want something like the below.

I admire the steadfastness I see in your coding effort.

Be well,
rir

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

my @data = (       # note parens not square brackets
   '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 $key (sort {$a cmp $b} keys %labels) {
    push @data, $labels{$key};
}

use Data::Dumper;
print Dumper @data;

__END__
$VAR1 = '17-02-2005';
$VAR2 = '18-02-2005';
$VAR3 = '19-02-2005';
$VAR4 = '20-02-2005';
$VAR5 = '21-02-2005';
$VAR6 = '22-02-2005';
$VAR7 = '23-02-2005';
$VAR8 = '24-02-2005';
$VAR9 =  [ 0, 0, 0, 0, 0, 0, 5, 4 ];
$VAR10 = [ 0, 0, 0, 0, 0, 0, 0, 0 ];
$VAR11 = [ 0, 0, 0, 0, 7, 0, 0, 2 ];
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