Update: Implemented the divisor suggestion by soonix.

Hi Discipulus,

To have Perl match the C output (regarding the number of characters), a missing append is needed at the end of the Perl script. The C code is found here and at stackoverflow. Regarding C, a fix is necessary described here (under Update 2) and here.

$pi .= $predigit;    # <-- missing line

print $pi =~ s/^03/3./r;
[download]

I validated using the following code, based on yours and replies by fellow monks. Notice the Perlish for my $k (0..$nines - 1) { ... } near the end of the script. In your demonstration, calling int on $predigit isn't necessary because $q contains an integer value; e.g. $q = int( ... ).

# The spigot algorithm for calculating the digits of Pi.
# http://www.cut-the-knot.org/Curriculum/Algorithms/SpigotForPi.shtml

use strict;
use warnings;

my $n   = 100;
my $len = int(10 * $n / 3) + 1;
my $pi;

my @a = (2) x $len;

my $nines    = 0;
my $predigit = 0;

for my $j (1..$n) {
    my $q = 0;

    for my $i (reverse 0..$len - 1) {
        my $x = 10 * $a[$i] + $q * ($i + 1);
        my $divisor = 2 * $i + 1;
        $a[$i] = $x % $divisor;
        $q = int($x / $divisor);
    }

    $a[0] = $q % 10;
    $q    = int($q / 10);

    if (9 == $q) {
        ++$nines;
    }
    elsif (10 == $q) {
        $pi .= $predigit + 1;
        for my $k (0..$nines - 1) {
            $pi .= 0;
        }
        $predigit = $nines = 0;
    }
    else {
        $pi .= $predigit;
        $predigit = $q;
        if (0 != $nines) {
            for my $k (0..$nines - 1) {
                $pi .= 9;
            }
            $nines = 0;
        }
    }
}

$pi .= $predigit;

print $pi, "\n";
[download]

Pi computed to 10,000 digits is found here.

Update: The above runs 1.4 times faster with small changes. Basically, use integer. Thanks, haukex.

2a3
> use integer;
5c6
< my $len = int(10 * $n / 3) + 1;
---
> my $len = 10 * $n / 3 + 1;
20c21
<         $q = int($x / $divisor);
---
>         $q = $x / $divisor;
24c25
<     $q    = int($q / 10);
---
>     $q    = $q / 10;
[download]

Regards, Mario