It's likely to mean exactly what it says. From perldiag:

       Use of uninitialized value%s
           (W uninitialized) An undefined value was used as if it were already
           defined.  It was interpreted as a "" or a 0, but maybe it was a
           mistake.  To suppress this warning assign a defined value to your
           variables.

           To help you figure out what was undefined, perl tells you what
           operation you used the undefined value in.  Note, however, that
           perl optimizes your program and the operation displayed in the
           warning may not necessarily appear literally in your program.  For
           example, "that $foo" is usually optimized into ""that " . $foo",
           and the warning will refer to the "concatenation (.)" operator,
           even though there is no "." in your program.