You could use closures, as has already been mentioned, or you could use tie. Something like this might work for you... I don't know.

    package Product;
    use Tie::Array;
    @ISA = qw/Tie::StdArray/;

    use strict;

    sub TIEARRAY {
        my $class = shift;
        bless [ @_ ], $class;
    }

    sub FETCH {
        my $them = shift;
        my $index = shift;
        return $index >= @$them ?
            eval join '*', @$them :
            $them->[$index];
    }

    package main;

    ## Initialize your array with 2 elements, 8 and 2
    tie my @p, 'Product', 8, 2;
    print $p[2], "\n";

    ## Change the second element to 5
    $p[1] = 5;
    print $p[2], "\n";

    ## You can even add new elements to the array; now
    ## you need to use $p[3] to get the product
    push @p, 10;
    print $p[3], "\n";
[download]

This is, admittedly, pretty hackish. But it works. :)

Output:

    16
    40
    400
[download]

If you try to access any element beyond the end of the array, it returns the product of the elements in the array. Admittedly, I don't really like those semantics much--anyone have any better ideas?