Well, the SQL solution would be to do whatever needs to be done to satisfy the SQL on the other side. (What you're getting is an SQL error, not a Perl one, after all).

The Perl solution would be to get rid of the cast, and simply test date_completed in Perl:

$sth=$dbh->prepare(select ....);
$sth->execute;
$res=$sth->fetchrow_hashref;
$$res{is_complete}=defined($$res_{date_completed});
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