I'm not too sure what your code is supposed to be doing. It looks a bit like you're trying to count the number of times a particular value occurs in an array. The normal way of doing this is to use a hash:

#!/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";
}
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But you might be trying to do something more complicated than that. Some more explanation of what you're trying to achieve would help