$field = $sth->fetchrow_array();

##</code><code>##

sub fetchrow_array {
    my @out;
    # ...
    if ( wantarray ) {               # list
        return @out;
    }
    elsif ( ! defined wantarray ) {  # void
        return;
    }
    elsif ( scalar @out <= 1 ) {     # scalar
        return $out[0];
    }
    else {                           # scalar
        die 'too many return values for scalar';
    }
}