I don't know what it means or how to avoid it, but the Devel::NYTProf POD mentions how you might be able to get more info:
Set trace level to N. 0 is off (the default). Higher values cause more detailed trace output.
I also used the CPAN grep feature to search for your warning, and it comes from the NYTProf.xs file.

In reply to Re: nytprof warning by toolic
in thread nytprof warning by erwan

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