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
in reply to Please help, (perl hash)
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.
$array[ ord( $town ) - 65 ][ $street - 1 ] = $houses;