Different query results in psql and Perl DBI with Postgres 7.2.1

I have a script that updates one record in a PostgreSQL 7.2.1 database
through the Perl DBI.

Here is the output from my log, which outputs the query sent to the
DBI and the return value:

---

Update query: UPDATE item SET status='SOLD' WHERE item_id=1129832
1 rows affected.

Update query: UPDATE item SET status='STORE' WHERE item_id=1130081
0E0 rows affected.

---

In both cases, the target record exists without question, and in the
latter case, I receive the expected "UPDATE 1" when I execute the same
query from within psql.

Even more perplexing: the "0E0" occurs ONLY when setting status to
'STORE', and not 100% of the time. (though it happens far more often
than not.) It seems to operate in stretches, too: when I am getting
1-row updates back from the latter query type, it happens several
times in succession.

Where do I even begin to attempt a diagnosis?