Okay, I should have been able to start deducing the comma thing by that point, too. I'm not sure why I was so oblivious to it. I did find the culprit however. I had nailed the problem down to 20 lines of code and your comment about a comma (makes absolute sense) lead me to the precise line that was glitching.
I'm embarrassed to admit this, but the following is the problem that was tripping me up. Note that the code has behaved exactly as wanted and expected for over a year with this line in it - go figure!
if (defined($duplicate_email_address), defined($matched_uid))
Obvious to anyone with a brain and a set of eyes (except myself apparently), the problem with the above line is that there is a comma where the && operand should be.
All I can offer in my defense is that I never suspected a problem like that because I'd have expected it to cause a syntax error or prevent compiling.
I can also now recall the exact step that lead me to this problem over a year ago when I wrote this new batch of code. I had copied the contents of something like my ($foo, $bar, $blah) = $sth->fetchrow_array; and then used that in a conditional and never went back to modify the syntax.
This was the last thing I was investigating. Every time I went back to check this code and see if I could find the cause of that error, I was looking for things more like a variable that was being defined but never used or something. Gah! So frustrating!
On a positive side, I rarely make the same mistake twice and I learned a thing or two from this thread. Thanks again, monks.