As noted in perlxstut, this particular example would only work on Unix as is:

   This extension is very Unix-oriented (struct statfs and the statfs 
+system call).  If you are not
   running on a Unix system, you can substitute for statfs any other f
+unction that returns multiple
   values, you can hard-code values to be returned to the caller (alth
+ough this will be a bit harder
   to test the error case), or you can simply not do this example.
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In other words, the storage size of buf cannot be determined because the struct statfs is not known on Windows (it's defined in sys/vfs.h on Unix).