sub process {
    my( $AoA,  $thingToProcess ) = @_;

    $AoA->[ $thingToProcess ][ 3 ] = $AoA->[ $thingToProcess ][ 1 ] 
                                   * $AoA->[ $thingToProcess ][ 2 ];
    ...
    return;
}

...

process( $AoA, 123 );