if ( $p{nanoseconds} ) {                          # <-- +0 should fix it
            $self->{nanoseconds} = $p{nanoseconds};
            $self->_normalize_nanoseconds;
        }
        else {
 
            # shortcut - if they don't need nanoseconds
            $self->{nanoseconds} = 0;
        }


... # and later

# make the signs of seconds, nanos the same; 0 < abs(nanos) < MAX_NANOS
# NB this requires nanoseconds != 0 (callers check this already)  # <-- GOTCHA!
sub _normalize_nanoseconds {

...