My program 

#!/usr/bin/perl -w

use strict;
use constant PI    => 4 * atan2(1, 2);

my $equatorial_radius = 6378.135; # equatorial radius, km
my $polar_radius      = 6356.750; # polar radius, km

# quadratic mean radius
my $qr = sqrt((3*($equatorial_radius^2))+($polar_radius^2)/4);

print "Earth's quadratic mean radius: ${qr} km\n";

# Volume of sphere is pi * radius^2
my $volume = PI * $qr^2;

print "Earth volume: $volume cubic km\n";

my $mcubed = 1000*1000*1000*$volume; 
print "Earth volume: $mcubed cubic metres\n";