#! /usr/bin/env perl
use strict;
use warnings;
my %numbers = qw/210 rg1000
                 212 rg1000
                 214 rg1000
                 215 rg1000
                 218 rg1003
                 221 rg1000
                 222 rg1003
                 223 rg1003
                 224 rg1003/;
my %rgs;
push @{$rgs{substr($_,0,2) . ':' .  $numbers{$_}}}, $_
  for keys %numbers;
 
# The first <=> will warn.
for (sort { $a <=> $b or @{$rgs{$b}} <=> @{$rgs{$a}} } keys %rgs) {
  /^(\d{2}):(.+)/;
  printf "%d => $2\n", @{$rgs{$_}} == 1 ? $rgs{$_}->[0] : $1;
}

__END__
$ ./mad
21 => rg1000
218 => rg1003
22 => rg1003
221 => rg1000