You'll need to require Exporter;

Wrong. You can load the Exporter modue at compile time, no need to delay until runtime, i.e. use Exporter instead of require Exporter.

and inherit

Wrong for any perl released since at least 2008. Inheriting from Exporter has side effects that may be unwanted. In particular, it will make Exporter's methods and function appear as methods of the class inheriting from Exporter. Prepare for really strange bugs if the class unintentionally inheriting from Exporter also inherits from another class that has methods with the same name as those inherited from Exporter.

Generally, importing the import() method from Exporer is sufficient and has no unwanted side effects.

See also Exporter in an OO module?, Advice on style.

before your @EXPORT will have any effect.

Wrong. OOP modules should not export methods, as others have explained.

Alexander

Wrong. ...
Wrong. ...
Wrong. ...

there are no sweeter words than "I told you so". ;-)

Yeah, I'm starting to see that about you :-)

Points well taken about require vs. use, and @ISA. The very first example in the Synopsis has long done what I need it to do. Maybe it's time for a documentation patch to de-emphasize the older calling convention? No reason for it to have top billing if it's not recommended for new code. Anyway, I'll definitely be adopting this advice going forward. Thanks.

OOP modules should not export methods, as others have explained.

Yes, I was the first to explain that, some hours before your reply.

Thank you for you help. I will try it.

 
package PDL::CV;

require Exporter;
our @ISA = qw(Exporter);
our @EXPORT = qw(rp);

sub rp {
    my ( $self, $input ) = @_;
    $self .= $input->reshape($self->dims);
    return $self;
}

sub new
{
    my $class = shift;
    my $self = {
        _input => shift,
    };
    bless $self, $class;
    return $self;
}

1;
[download]

Ah, I completely missed the fact that you're mixing Exporter with OO programming. My mistake for not spotting that sooner. But the error message you're getting has nothing with Exporter, because object methods don't need to be (and should not be) exported, because there's no reason to pollute the caller's namespace when object methods can be called regardless. The real problem is evident in the error message you get:

Can't locate object method "rp" via package "PDL" at test_pdl6.pl line 27.

Is for the class "PDL", rather than your "PDL::CV" package where the rp method is defined. The problem must lie with your test (caller) code in test_pdl6.pl, because aside from the needless use of Exporter, your package's code looks reasonable.

This should have been my top level reply.

Thank you so much for your kind reply. I understand your reply. I think I should change my question, is: can we add new methods to a exist object in a package?

OK, Thank you so much !