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".