~/perl$ cat func_test.pl 
#!/usr/bin/perl 

#sum of number between 1 and 1,000,000
$n = 1_000_000;

print addup($n, 0);

sub addup
{
    return $_[1] if !$_[0];
    return addup($_[0]-1,$_[1]+$_[0]);
}

~/perl$ cat loop_test.pl 
#!/usr/bin/perl 

#sum of number between 1 and 1,000,000
$n = 1_000_000;
$sum = 0;

$sum += $n-- while($n);


~/perl$ time ./func_test.pl 
500000500000
real    0m4.025s
user    0m3.568s
sys     0m0.455s

~/perl$ time loop_test.pl  

real    0m0.390s
user    0m0.388s
sys     0m0.001s