The difference between

    # broomduster
    my $h = $a{$state}{$city};
    $h->{'Color'} = $color;
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and

    # ikegami
    my $r = \( $a{$state}->{$city} );
    ${ $r }->{Color} = $color;
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is that only the latter autovivifies if necessary. For example, here's what happens if

$a{'WI'}{'GreenBay'}{'Color'} = 'Black';
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# broomduster
$VAR1 = {
          'WI' => {}
        };
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# ikegami
$VAR1 = {
          'WI' => {
                    'GreenBay' => {
                                    'Color' => 'White'
                                  }
                  }
        };
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The following would also do:

    my $h = $a{$state}{$city} ||= {};
    $h->{'Color'} = $color;
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Thanks ikegami,

autovivification was an issue -- I started out using broomduster's method, but set the sub-hash to '{}' if the hash keys didn't exist. Your method is more compact.

Thanks, Matt