As mentioned to samtregar above, I misrepresented that the die was caught "by" the parent -- in fact, it was caught in the child, by code that never was intended to have been forked.

This is all on Debian Sarge, with Perl 5.8.4 and threads enabled. But I don't think any of it is platform-specific: once I understood what the code was underneath, it became clear that things were going by the book.