#!/usr/bin/perl

use warnings;
use strict;

my @ID = qw/1 1 1 1 1 1 1 2 2 2 2 2 3 3 4 4 4 4 5 5 5 5 6/;

my %counts;
foreach my $id (@ID) {
        $counts{$id}++;
}

while(my ($num, $count) = each(%counts)) {
        print "$num : $count\n";
}