#!/usr/bin/perl -w
use strict;
use Data::Dumper;

my @ID = (1,1,1,1,1,1,1,1,2,2,2,2,2,2,3,3,4,4,4,4,4,5,5,5,5);

#
# count
#
my %items =();
$items{$_}++ for @ID;

print "$_ => $items{$_}\n" for sort keys %items;

#
# detail
#
%items =();
my $count =0;
push @{$items{$_}}, $count++ for @ID;

print Data::Dumper->Dump([ \%items], ['items']);

__END__
output:
1 => 8
2 => 6
3 => 2
4 => 5
5 => 4

$items = {
          '4' => [
                   16,
                   17,
                   18,
                   19,
                   20
                 ],
          '1' => [
                   0,
                   1,
                   2,
                   3,
                   4,
                   5,
                   6,
                   7
                 ],
          '3' => [
                   14,
                   15
                 ],
          '2' => [
                   8,
                   9,
                   10,
                   11,
                   12,
                   13
                 ],
          '5' => [
                   21,
                   22,
                   23,
                   24
                 ]
        };