The following works for the sample data given.

use warnings;
use strict;

my %numbers = ( '210' => 'rg1000',
                '212' => 'rg1000',
                '214' => 'rg1000',
                '215' => 'rg1000',
                '218' => 'rg1003',
                '221' => 'rg1000',
                '222' => 'rg1003',
                '223' => 'rg1003',
                '224' => 'rg1003' );

my %groups;
my %hits;

for (sort keys %numbers) {
    my $part = substr $_, 0, 2;
    my $group = $numbers{$_};
    
    if (exists $hits{"$part,$group"}) {
        # We've seen this prefix with this group before
        delete $groups {$hits{"$part,$group"}};
        delete $groups {$hits{"$part,$group"}.",$group"};
        $groups{"$part,$group"} = "$part => $group";
    } elsif (exists $hits{$part}) {
        # new group for existing prefix
        $groups{"$_,$group"} = "$_ => $group";
        $hits{"$part,$group"} = $_;
    } else {
        #new prefix
        $hits{$part} = $_;
        $hits{"$part,$group"} = "$_,$group";
        $groups{"$_,$group"} = "$_ => $group";
    }
}

foreach my $item (sort keys %groups) {
        print "$groups{$item}\n";
}
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Prints:

21 => rg1000
218 => rg1003
22 => rg1003
221 => rg1000
[download]