Hmmm. I think the I found something odd. I added three debugging lines:

C:DBI:AS:search_where: (line 31)
  warn "search_where"; 
  warn Dumper ($phrase, \@bind);

C:DBI:retrieve_from_sql: (line 1067)
  warn "retrieve_from_sql"; 
  warn Dumper $sql, \@vals;

C:DBI:sth_to_objects: (line 1125)
  warn "sth_to_objects"; 
  warn Dumper $args;
[download]

At each one, I dumped out all the params being worked on. Here's what happens in the page where the problem is consistant... The page loops over multiple items, and it always breaks on the 3rd item. First iteration:

search_where -
$VAR1 = '( ID = ? )';
$VAR2 = [
          '3225-1'
        ];

retrieve_from_sql -
$VAR1 = '( ID = ? )';
$VAR2 = [
          '3225-1'
        ];

sth_to_objects -
$VAR1 = [
          '3225-1'
        ];
Attempt to free unreferenced scalar: SV 0x88020e4
[download]

Second iteration:

search_where -
$VAR1 = '( ID = ? )';
$VAR2 = [
          '5912-1'
        ];

retrieve_from_sql -
$VAR1 = '( ID = ? )';
$VAR2 = [
          '5912-1'
        ];

sth_to_objects -
$VAR1 = [
          '5912-1'
        ];
Attempt to free unreferenced scalar: SV 0x880e61c
[download]

Third iteration (and this one breaks):

search_where -
$VAR1 = '( ID = ? )';
$VAR2 = [
          '5913-1'
        ];

retrieve_from_sql -
$VAR1 = '( ID = ? )';
$VAR2 = \'5913-1';

sth_to_objects -
$VAR1 = \'5913-1';

MyApp::Sets can't SELECT id, id, setnumber, setrev, name, theme, year,
+ pcs, figs, picture, msrp, instructions, inventory
FROM   SETS
WHERE  ( ID = ? )
: Not an ARRAY reference
[download]

Looking at this, it's obvious that for some reason, the 3rd run does change from an arrayref to a scalarref. But I can't see why, or how this happens? There is _nothing_ touching that value between the 2nd debug line and the 3rd debug line. Makes me think it's some sort of thread-safe issue... Except that this particular CGI is just running plain perl - not SpeedyCGI or anything else. How can I dig deeper?