Does

void do_simulation() {
    struct s* m;
    m = (struct s*)malloc(10000000 * sizeof(struct s));
    free(m);
    m = (struct s*)malloc(10000000 * sizeof(struct s));
    free(m);
}
[download]

use more memory than

void do_simulation() {
    struct s* m = (struct s*)malloc(10000000 * sizeof(struct s));
    free(m);
}
[download]

It could be that the memory is being pout back into Perl's free memory pool and you just don't realize it.