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First, my comment about using qr// was specific about when writing modules. If you are writing modules then you should consider other peoples' environments. So I think the minor effort to support what are still quite common versions of Perl to be good manners.
After all of the above, I am even more convinced that:
The documentation for Carp.pm is quite simple, saying that the error will be reported as coming from the caller (not from the caller of the caller of the caller etc. for as long as it takes to get out of your inheritance heirachy). I think this is a much better idea.
My first run-in with Carp.pm skipping more than one level of the call stack kinda made sense since it allowed your module to have a helper routine that didn't check all of its arguments closely and could pass on bad arguments to an underlying routine and in that specific case, it makes sense to have the error report two levels up. Well, sorta. At least it doesn't help much to have an error reported in your helper routine.
I say "sorta" because I'd better be pretty careful in my underlying routine to word my error message clearly. It doesn't do much good to have code fred("bar") report "_fetchField() called with undefined field name". So I think that in most cases, even the best case I that can come up with, skipping more than one level of call stack doesn't really give a better message than just simply always skipping exactly one level would do.
I think it makes much more sense to let those specific error messages (that are very carefully worded and carefully chosen such that they really were caused more than one level up and will be understood when reported more than one level up) should call carp() in such a way that carp() knows how far up the stack to climb.
Now skipping the whole inheritance heirarchy is just madness. So everyone who uses HTML::ParseTree (or whatever some module is that expects the user to use inheritance in order to make use of the module) should be getting their programming errors popped up to the next higher package instance?
I can see wanting carp() to skip a couple levels of inheritance heirarchy in very rare cases where I have several packages that are developed together, probably in a single source file, and some specific arguments are not checked until we get a few levels deep. Then only errors about those specific arguments should be allowed to bubble up the appropriate level.
But just because I inherit from a class certainly doesn't mean that I should avoid checking my arguments for validity before passing them along to that class. But this seems to be what the designers of Carp assume, no? If I inherit from someone else's package, I really do want to be told when I use their package wrong. I will write my code such that errors passed in by users of my code will be detected by my code, not blindly passed on to the other module.
So the fact that Exporter has a "bug" in that it doesn't take into account the bad design flaw of Carp and this bug can be worked around by making the inheritance tree inconsistant over time, isn't really a good argument for encouraging such inconsistancy.
I think you may be too caught up in the whole Carp mess. (:
And all of this just enforces my commitment that lots of things go on between declaring @ISA and run-time initializing @ISA and it doesn't make sense to leave your inheritance in an inconsistant state during all of this processing.
Sorry, I don't have time right now to dig up the easy way to break things when you don't use BEGIN or use base. But several other people got convinced enough that patches were issued such that BEGIN blocks were actually documented as the proper way to initialize your package variables like $VERSION and @ISA.- tye (but my friends call me "Tye")