I recommend the Win32::DirSize module, by yours truly. It was written to calculate directory sizes, but handles whole drives as well. Here's a sample:

use strict;
use Win32::DirSize;

my $DiskInfo;  # this stores the disk information
my $Result = disk_space(
   "C:",
   $DiskInfo,
);

if ($Result == DS_RESULT_OK) {

   my ($DiskSizeUnit, $DiskFreeUnit, $QuotaFreeUnit);
   my ($ConvDiskDize, $ConvDiskFree, $ConvQuotaFree);

   $ConvDiskDize = best_convert(
     $DiskSizeUnit, 
     $DiskInfo->{HighTotalBytes}, 
     $DiskInfo->{LowTotalBytes},
   );
   $ConvDiskFree = best_convert(
     $DiskFreeUnit, 
     $DiskInfo->{HighFreeBytes}, 
     $DiskInfo->{LowFreeBytes},
   );
   $ConvQuotaFree = best_convert(
     $QuotaFreeUnit, 
     $DiskInfo->{HighQuotaBytes}, 
     $DiskInfo->{LowQuotaBytes},
   );

   print "Disk Size = $ConvDiskDize $DiskSizeUnit \n";
   print "Disk Free = $ConvDiskFree $DiskFreeUnit \n";
   print "Quota Free = $ConvQuotaFree $QuotaFreeUnit \n";
 }
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