I guess that your entire program is a bit more complex than the bits you posted.

yes, treat it as pseudocode. I can't create proof-of-concept code.

Perhaps a $SIG{__DIE__} handler would reveal more:

Yes, yo're right. fold_constants mostly happens, when child terminated with exception. However. All 'die's are inside eval (you can see eval in pseudocode). After eval caught, program executes dump_error() and exits (this way I see all errors) Also I did not see the panic, until I installed signal handlers into "children" code above.