#!/usr/bin/perl -w                             

use strict; # always good

unshift(@INC,"$ENV{'PWD'}");
  use List::Util qw(max) ;  # I could not find a "Util", but this one 
+seems fine ;-)

  my $file_name;
  my $ans;      

  if (@ARGV == 1) {
     chomp ($file_name=$ARGV[0]);
  }                              
  else {                         
   print "\n\nPlease enter the file name to certify: ";
   chomp ($file_name=<STDIN>);                         
   while(1) {                                          
      print "\n\nYou entered $file_name - is this correct? <y or n>:";
      chomp ($ans=<STDIN>);                                           
      if ($ans =~ /[Nn]/) {                                           
         print "\n\nPlease enter the server name: ";                  
         chomp ($file_name=<STDIN>);                                  
         next;                                                        
      }                                                               
      elsif ($ans =~ /[Yy]/) {                                        
         last;                                                        
      }                                                               
      else {
         next;
      }
    }
  }
  open(STUFF,"<$file_name") or die "$!";

  my %number_ids = () ;
  my $no_of_groups = 0;  # used to count the number of groups of that 
+file

  while (my $line = <STUFF>) {
    $line =~ s/\s+\z// ;
    $line =~ s/\A\s+// ;
    my @csv = split(/:/, $line) ;

    if (defined($csv[1]) && ($csv[1] =~ m{((CO)\d{5})})) {
      my $num = $1;        # my necessary due to strictures
      $number_ids{$num}++;
          $no_of_groups++; # increment Group Counter
    }
    elsif (defined($csv[0]) && ($csv[0] =~ m/^Group.*/i)) {
      show_most_popular(\%number_ids);
      print "$csv[1],";
      %number_ids = ();
    }
    else {
      # what to do we do with peculiar lines?
    };
  };

  show_most_popular(\%number_ids);

  sub show_most_popular {
    my ($r_ids) = @_ ;

    return if !%$r_ids ;

    my $max = max(values %$r_ids) ;
    my @popular = () ;
    while (my ($id, $count) = each %$r_ids) {
      if ($count == $max) { push @popular, $id ; } ;
    } ;

    print join(',', sort @popular), " appeared most and $max out of $n
+o_of_groups times\n";  # output as requested ;-)
  } ;
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