It means that one (or more) Tk variables was (unnecessarially and incorrectly) cloned from the main thread into the child thread when you spawned it. And when Perl attempts to clean it up (during global destruction), it doesn't know how. It can be safely ignored.

If you only intend to start one (or a few) threads, you can defer the error until the program terminates by not detaching the threads. You will then receive a message along the lines of:

Perl exited with active threads:
        0 running and unjoined
        1 finished and unjoined
        0 running and detached
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Which again can be safely ignored (but unfortunately not turned off).

As zentara pointed out elsewhere, it is possible to avoid these errors by starting the non-Tk threads prior to loading Tk so that the cloning doesn't happen, but unless you are intending starting enough threads for their dead corpses hanging around to become a (memory) problem, the extra complexity involved is usually not worth the effort.