#!/usr/bin/perl
use strict;

my %idcount;

while (<>) {
    if ( /,CO(\d+)\s/ ) {
        $idcount{$1}++
    }
}
my ( $highest ) = sort { $idcount{$b} <=> $idcount{$a} } keys %idcount;
my $total = 0;
$total += $_ for ( values %idcount );

print "Highest count was for group CO$highest, with $idcount{$highest} out of $total users\n";

##</code><code>##

Highest count was for group CO12345, with 4 out of 6 users

##</code><code>##

my ($maxcount) = sort { $idcount{$b} <=> $idcount{$a} } keys %idcount;
my $highest = join(",CO", grep { $idcount{$_} == $maxcount } keys %idcount );