Normally the hash of hash as stated. But there is interestingly an array of array option for this very specific case where the towns and streets are nameless and have a contiguous identification system. For example

$array[ ord( $town ) - 65 ][ $street - 1 ] = $houses;
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the expression involving ord converts the town letter into its corresponding position in the outer array and the inner array needs only the subtraction of one to map streets 1-9 to their array indexes in Perl.