my ($field1, $field2, $field3) = (undef, undef, undef);
my $sth = $dbh->prepare('select * from table');
$sth->execute;
if ($sth->rows > 0) {
    $sth->bind_columns(\($field1, $field2, $field3));
    $sth->fetch;
}
$sth->finish;
for ($field1, $field2, $field3) { $_ ||= '' }

##</code><code>##

    $dbh->do('insert into blah (?, ?, ?)', undef, $x, $y, $z);
    if ($dbh->err) {
        if ($dbh->err == 1062) {
            # duplicate
        } else {
            # some other problem
        }
    }