You didn't post code that would let me reproduce your problem, so I can only guess.

One guess is that you wrote another sub _stringify in a child class, and expected it to be called by the string overloading.

It won't, of course, because you gave overload a reference to this one subroutine. If you want that kind of polymorphism, use

use overload '""' => sub { shift->_stringify() };
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If you don't, make sure to do an apple-to-apple comparison by writing $logger->trace("Zep is " . ref($zep) . " stringifies as $zep" . " but _stringify returns " . OT::Zeppelin::_stringify($zep) );