computing pi in Perl 6

pi-agm.pl

use v6;

constant number-of-decimals = 100;

sub my-sqrt($n) {
    my Int $square = (10**(2*number-of-decimals) * $n).round;
    return FatRat.new: .[*-1], 10**number-of-decimals given
    1, { ($_ + $square div $_) div 2 } ... * == *
}

my ($a, $n) = 1, 1;
my $g = my-sqrt(1/2);
my $z = .25;

for ^10 {
    given [ ($a + $g)/2, my-sqrt($a * $g) ] {
	$z -= (.[0] - $a)**2 * $n;
	$n += $n;
	($a, $g) = @$_
    }
}
say $a ** 2 / $z;