## Use meaningful names obviously!!
use constant { 
    0 => FIRST, 1 => SECOND, 2 => THIRD, 3 => FOURTH, 
    4 => FIFTH, 5 => SIXTH, 6 => SEVENTH 
};

sub process( 
    our @s;
    local *s = shift;
    $s[ FOURTH ] = $s[ SECOND ] + $s[ THIRD ];
    ...
    return;
}    

process( $AoA[ $thingToProcess ] );