use strict;
use warnings;

use Data::Dumper;

my %a;
$a{'WI'}{'GreenBay'}{'Color'} = 'Black';

foreach my $update ("WI,GreenBay,White") {
    my ($state,$city,$color) = split /,/, $update;
    my $h = $a{$state}{$city};
    $h->{'Color'} = $color;
};

print Dumper(\%a), "\n";
__DATA__
$VAR1 = {
          'WI' => {
                    'GreenBay' => {
                                    'Color' => 'White'
                                  }
                  }
        };