Check out the Win32::API module. It's not included with ActiveState's perl distribution, but you can install it using ActiveState's PPM tool. From the documentation:

  use Win32::API;
  $function = new Win32::API($library, 
                             $functionname, 
                             \@argumenttypes, 
                             $returntype);
  $return = $function->Call(@arguments);
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Update:

To effectively use this method you will also need the API documentation. Here's the prototype for the GetDiskFreeEx function:

BOOL GetDiskFreeSpaceEx(
  LPCTSTR lpDirectoryName,  // pointer to the directory name
  PULARGE_INTEGER lpFreeBytesAvailableToCaller, // receives the number
+ of bytes on
                                                // disk available to t
+he caller
  PULARGE_INTEGER lpTotalNumberOfBytes,    // receives the number of b
+ytes on disk
  PULARGE_INTEGER lpTotalNumberOfFreeBytes // receives the free bytes 
+on disk
);
[download]

This function lives in KERNEL32.DLL.

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Coyote