All of those represent a good cross-section of challenges that always seem to make me think "this is a *really* hard challenge!"

One other that, at least for me, is inevitably "hard" is when I'm presented with complicated data content in a file(s) that just won't yield to straightforward information extraction. They usually have a distinct lack of tags or many different "keywords" for specific items; they usually have lots of numbers intermixed with lots of miscellaneious text.

Of course, this is where Perl shines...it is, afterall, a superb data extraction language. But inevitably, I have large mixtures of parsing, string manipulations, and regexs that seem to be convoluted and difficult to keep straight. I suspect a lot of the challenge comes from being insufficiently adept and experienced with Perl and it's multitude of capabilities.

But whatever the cause, information extraction from complicated source data structures always seems to be nightmarish for me.