gistfile1.PL

package Session;

use MooseX::POE;
use namespace::clean -except => 'meta';

has session_id => (
    isa => 'Int',
    is  => 'rw',
);

sub START {
    my $id = POE::Kernel->get_active_session->ID;
    $_[OBJECT]->session_id($id);
}

package main;

use Moose;
use POE;

POE::Session->create(
    package_states => [
        (__PACKAGE__) => ['_start'],
    ],
);
POE::Kernel->run;

sub _start {
    my $s = Session->new();
    # $s->session_id is still undefined
    print $s->session_id, "\n";
}