Yes. That is, instead of all the code in the OP, you would want something like this: 
#!/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";

[download]
For the sample data in the OP, that prints: 
Highest count was for group CO12345, with 4 out of 6 users

[download]
(which I got by running the script in a terminal window, then pasting the 6 lines of data into the window, to emulate "typing" on stdin; the same result would happen by piping those six lines to the perl script from some other process, or storing them in a file and giving the file name as the sole command-line arg when running the script).

UPDATED to include declaring, computing and printing "$total" (as per the OP spec).

Oops... another update -- forgot about the case of possible ties for "highest". So, in place of the one line above that declares/assigns a value to $highest, you would need these two lines, which will set $highest to be a string of one or more comma-separated group-ids (including "CO" before the digits of non-initial id values): 

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

[download]
Nothing else needs to change. Then replace "$idcount{$highest}" with "$maxcount" in the print statement.

In reply to Re^4: Count occurences of numbers by graff
in thread Count occurences of numbers by walkingthecow

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