If the file is not too big, slurp it into $_ and do a global regex match to get each "ITEM" and attendant lines which you can then split on newline. Like this

use strict;
use warnings;

use Data::Dumper;

{
    local $/;
    $_ = <DATA>;
}

my $raDataItems =
   [
       map {[split m{\n}]}
       m{(ITEM.*?)(?=(?:\z|ITEM))}sg
   ];

my $dd = Data::Dumper->new([$raDataItems], [q{raDataItems}]);
print $dd->Dumpxs();

__END__
ITEM NO:1
[aaa]
111
[bbb]
222
[ccc]
333
ITEM NO:2
[ddd]
444
[eee]
555
[fff]
666
ITEM NO:3
[ggg]
777
[hhh]
888
[iii]
999
[download]

and the output is

$raDataItems = [
                 [
                   'ITEM NO:1',
                   '[aaa]',
                   '111',
                   '[bbb]',
                   '222',
                   '[ccc]',
                   '333'
                 ],
                 [
                   'ITEM NO:2',
                   '[ddd]',
                   '444',
                   '[eee]',
                   '555',
                   '[fff]',
                   '666'
                 ],
                 [
                   'ITEM NO:3',
                   '[ggg]',
                   '777',
                   '[hhh]',
                   '888',
                   '[iii]',
                   '999'
                 ]
               ];
[download]

I hope this is of use.

Cheers,

JohnGG