use Win32::API; 

sub make_large_int {
   my ($lo, $hi) = @_;
   return $hi * (1 + 0xffffffff) + $lo;
}

# Return a two-element list: (size, free) in bytes.
sub win_get_disk_free {
   my $dir = shift;   # $dir is a directory on the disk of interest
   $dir =~ tr{/}{\\};
   my $GetDiskFreeSpaceEx = Win32::API->new(
                               'kernel32.dll',
                               'GetDiskFreeSpaceEx',
                               ['P','P','P','P'], 'I')
      or die;
   my $lpFreeBytesAvailable     = pack('L2', 0, 0);
   my $lpTotalNumberOfBytes     = pack('L2', 0, 0);
   my $lpTotalNumberOfFreeBytes = pack('L2', 0, 0);
   $GetDiskFreeSpaceEx->Call($dir,
                             $lpFreeBytesAvailable,
                             $lpTotalNumberOfBytes,
                             $lpTotalNumberOfFreeBytes)
      or die "GetDiskFreeSpaceEx failed ($^E)\n";
   return ( make_large_int(unpack('L2', $lpTotalNumberOfBytes)),
            make_large_int(unpack('L2', $lpTotalNumberOfFreeBytes)) );
}

my ($size, $free) = win_get_disk_free('C:/');
printf "disk size = %d MB\n", $size/(1024*1024);
printf "disk free = %d MB\n", $free/(1024*1024);