To be honest, I can't work out how it gets in there. The 100+ *alloc macros and functions in the Perl sources make it impossible to work out what is going on at the source level.

And the use of C++ (style) classes for odd bits of the code, with runtime built dispatch tables throws my debugger's stack trace into loops. That means I can't follow it in that way. I can only tell you where it appears to be trapping.