use Data::Dumper;

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

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

print Dumper(\%a), "\n";