functional perl result
sub successor($k){
$k+1;
}
sub cube($k) {
$k**3;
}
#sub sub_cubf($n){
# sum($n,successor,cube);
#}
sub natural($k) {
$k;
}
sub pi($k) {
8/($k*($k+2));
}
sub pi_term($k) {
$k+4;
}
say sum_natural(1000);
say sum(1000,&natural,&successor);
say sum_pi(2e3);
say sum(2e3,&pi,&pi_term);
say sum_cube(10);
say sum(10,&cube,&successor);