Consider this:

use strict;

package Foo;

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

sub _init {
    my $self = shift;
    $self->{important_flag} = 1;
}

package Foo::Bar;
use base 'Foo';

sub new {
    my $class = shift;
    my $self = $class->SUPER::new(@_);
    $self->_init('har har har');
    return $self;
}

sub _init {
    my $self = shift;
    $self->{ext_prop} = $_[0];
}

package main;
use Data::Dumper;

my $foo = Foo->new;
my $bar = Foo::Bar->new;
print Dumper([$foo, $bar]);

__END__

$VAR1 = [
          bless( {
                   'important_flag' => 1
                 }, 'Foo' ),
          bless( {
                   'ext_prop' => 'har har har'
                 }, 'Foo::Bar' )
        ];
[download]

The author of Foo::Bar probably didn't know about Foo's &_init, and shouldn't have needed to either. Private methods are private methods. If you want your method to be overridable you shouldn't use a leading underscore and you should document it. Using method invocation on private methods isn't a problem for the module author, it's a problem for those that try to inherit it. If I use a leading underscore on a method in my module I expect that no one else runs it -- that counts for both deriving and derived classes. I also expect no one to override it. If I wanted that I wouldn't use a leading underscore.

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