Did you even read the "solved" post? I did everything by the book in terms of my program - I output the query before running it so I knew exactly what was going to the database, I wrote and re-wrote the query a hundred times. And as far as I could tell the advice of the monks consisted of

A) Post some code (which I did), and
B) Use DBI->trace(), which I did, and was the solution I was looking for.

As to my assertion that the error is in the DBI code, and not my own, here's the error I was getting in the Apache logs (using $dbh->{ShowErrorStatement}, which according to the documentation prints out the offending query):

image_admin.pl:
...You have an error in your SQL syntax near '' at line 1 [for stateme
+nt ``SELECT parent_id FROM categories WHERE id=1'']) at ...
[download]

The error was in fact in a different query:

UPDATE categories SET name='some name', parent_id=2 WHERE id=

Had that been in the error logs, I wouldn't have had a problem finding it. So it's an error either in the DBI error logging, or in the DBI documentation of ShowErrorStatement.

I understand people who are leery of assertions that just-written code is better than an established codebase, but I feel I have a good case here.