Just to give a slight hint at how you can use closures to hide methods:

#!/usr/bin/perl -w
use strict;
package Foo;

{
    # private
    my $set_a = sub {
        my $self = shift;
        $self->{a} = shift;
    };

    # public
    sub a {
        my $self = shift;
        return $self->{a};
    }

    # public
    sub new {
        my $self = bless {},shift;
        $self->$set_a('abcdef');
        return $self;
    }
}

package main;

my $foo = Foo->new;
print $foo->a();
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It is a pretty secure and not too hard way to hide private methods, but I've never used it in production code, because I happen to agree with the philosophy that undocumented methods are private. It does have the advantage of not polluting namespaces with private methods, though.