The google results say that it is likely a flaw in something generated by h2xs.
The reason you didn't get details about where the warning came from is because you overrode the __WARN__ handler, probably by doing "use diagnostics;". So comment out that line and you'll likely be told not only what *.ph file is generating the warning but also what module pulled that file in (and what module pulled that module in, etc.).
(This is yet another example of why it might be a very good idea for eval to suppress warnings, especially "mandatory warnings".)