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my ($field1, $field2, $field3) = (undef, undef, undef);
my $sth = $dbh->prepare('select * from table');
...
}
$sth->finish;
for ($field1, $field2, $field3) { $_ ||= '' }

    $dbh->do('insert into blah (?, ?, ?)', undef, $x, $y, $z);
    if ($dbh->err) {
...
            # some other problem
        }
    }