Not sure I've understand the problem. You want the sum of the products of a set of pairs, right?

If so, you don't need a hash, you could do:

use strict;
use warnings;

my @change = (1.15, -0.1, 5.4, 1.03, -0.241);
my @numberofshares = (100, 400, 200, 300, 240);

my $sum=0;
while (my $k=pop @change) {
  $sum+=$k*pop @numberofshares;
}
print $sum;
[download]

Warning: untested