I'm neither an expert on C++ nor Java.

Could you please explain the semantic difference to this demo of private methods achieved with my ?

use strict;
use warnings;
use 5.10.0;


# ========================================
#          class definition
# ========================================

{
    package Class;

    use Carp qw/cluck/;
    use Data::Dump qw/pp dd/;

    # ----- constructor

    sub new {
        my ($class,@init)=@_;

        return bless {@init}, $class;
    }

    # ----- private methods

    my $private = sub {
        my ($self) = @_;

        cluck "I'm a PRIVATE METHOD of OBJ= " . pp $self ;
    };


    # ----- public methods
    sub public {
        my ($self) = @_;

        cluck "I'm a PUBLIC METHOD of OBJ= " . pp $self ;
        warn " ... calling private methods...\n";

        $self->$private();
    }

}



# ========================================
#                 demo
# ========================================

package main;

my $obj = Class->new( ID => 1 );
my $obj2 = Class->new( ID => 2 );

$obj->public;
$obj2->public;



$obj->private;       # fails
#$obj->$private;     # doesn't compile
[download]

-*- mode: compilation; default-directory: "d:/tmp/pm/" -*-
Compilation started at Sat Dec 19 16:53:21

C:/Perl_524/bin\perl.exe -w d:/tmp/pm/private_method.pl 
I'm a PUBLIC METHOD of OBJ= bless({ ID => 1 }, "Class") at d:/tmp/pm/p
+rivate_method.pl line 37.
    Class::public(Class=HASH(0x1cc1e0)) called at d:/tmp/pm/private_me
+thod.pl line 56
 ... calling private methods...
I'm a PRIVATE METHOD of OBJ= bless({ ID => 1 }, "Class") at d:/tmp/pm/
+private_method.pl line 29.
    Class::__ANON__(Class=HASH(0x1cc1e0)) called at d:/tmp/pm/private_
+method.pl line 40
    Class::public(Class=HASH(0x1cc1e0)) called at d:/tmp/pm/private_me
+thod.pl line 56
I'm a PUBLIC METHOD of OBJ= bless({ ID => 2 }, "Class") at d:/tmp/pm/p
+rivate_method.pl line 37.
    Class::public(Class=HASH(0x1cc0d8)) called at d:/tmp/pm/private_me
+thod.pl line 57
 ... calling private methods...
I'm a PRIVATE METHOD of OBJ= bless({ ID => 2 }, "Class") at d:/tmp/pm/
+private_method.pl line 29.
    Class::__ANON__(Class=HASH(0x1cc0d8)) called at d:/tmp/pm/private_
+method.pl line 40
    Class::public(Class=HASH(0x1cc0d8)) called at d:/tmp/pm/private_me
+thod.pl line 57
Can't locate object method "private" via package "Class" at d:/tmp/pm/
+private_method.pl line 61.

Compilation exited abnormally with code 255 at Sat Dec 19 16:53:22
[download]

Well, one major difference is that that $private is not a member of the class at all, it is a lexical in the same block in which you've defined the subs of the class. And, of course, it's singleton. A class private in C++/Java is not singleton. It could be declared 'static', which would make it singleton; but that still wouldn't be the same as what you've shown. If your $private were truly equivalent, you'd be able to write this:

use strict;
use warnings;

{
    package Class;

    my $n = 42;

    sub new {
        my $pkg = shift;
        bless {}, $pkg;
    }

    sub foo;
}

sub Class::foo
{
    print "$n\n";
}
[download]

But you can't, of course, because $n is out of scope at that point. It is not a member of the "class".

I reckon we are the only monastery ever to have a dungeon stuffed with 16,000 zombies.

> But you can't, of course, because $n is out of scope at that point. It is not a member of the "class".

I was aware of the scoping/closure effect, but I very rarely see Perl code where class definitions are dispersed over more the one file.

I.O.W file scope is sufficient.

And I think we aggree that Perl allows so many kinds of introspection (e.g. PadWalker) that privacy becomes relative.

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery}

The term "private variable" certainly has an established meaning in Perl, it appears many times in the perldocs for my-variables.