use strict ; use warnings ;
  use List::Util qw(max) ;

  open my $STUFF, "<", "stuff.csv"   or die "$!" ;

  my %number_ids = () ;

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

    if    (defined($csv[2]) && ($csv[2] =~ m/^\d+$/)) {
      $number_ids{$csv[2]}++ ;
    }
    elsif (defined($csv[4]) && ($csv[4] =~ m/^group\:/i)) {
      show_most_popular(\%number_ids) ;
      print $line, "\n" ;
      %number_ids = () ;
    }
    else {
      # OK, so what 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), "\n" ;
  } ;