D:\>echo Can't call method "Filesys::DiskSpace::df" without a package or object reference at ./try.pl line 27. | splain
Can't call method "Filesys::DiskSpace::df" without a package or object
        reference at ./try.pl line 27.  (#1)
    (F) You used the syntax of a method call, but the slot filled by the
    object reference or package name contains an expression that returns a
    defined value which is neither an object reference nor a package name.
    Something like this will reproduce the error:

        $BADREF = 42;
        process $BADREF 1,2,3;
        $BADREF->process(1,2,3);