By all means let me write your code for you..

print "Free space: " . (split(/\s+/, qx(dir c: | find /I "bytes free")
+))[3];

=pod
Output:
Free space: 29,143,982,080
=cut
[download]

more seriously 'perldoc Win32::Perflib' is a viable means of getting aproximately what you ask.

Update: read too quickly, you wanted total drive space. Can still be calculated indirectly using the perflib, but I guess the backticks example is bad.