1+1=0

     sub preallocate {
         my (@c);
         $#c = 99999;
         for (my $i; $i < 100000; $i++) {
             $c[$i] = rand;
         }
     }

The answer was eventually provided by Chip Salzenberg
Perl has a clever optimization in it
Perl figures that @c got big once, so it is likely to get big again
When preallocate returns, @c is not deallocated
- The next call re-uses the same space as the last call
And that leaves $#c = 99999 with nothing to do
- In fact, it's a small waste of time because it's superfluous
So you pay the cost for your 'optimization'
- But the gross benefit is zero because you already had the benefit
One optimization plus one optimization looks like zero optimizations