package Foo;

use Moose;

has 'y' => (
    is => 'Num',
    is => 'rw'
);

has 'x' => (
    is => 'Num',
    is => 'rw'
);

sub init_y {
    my ($self, $value) = @_;
    $self->y(sqrt($self->x));
}

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

    print STDERR "Foo::BUILD running\n";
    $self->init_y;
}

1;
##</code><code>##

package Foo::Bar;

use Moose;
extends 'Foo';

before 'BUILD' => sub {
    my $self = shift;
    print STDERR "Foo::Bar before BUILD running\n";
    $self->x(25);
};

1;
##</code><code>##

use lib '.';
use Foo::Bar;

use Data::Dumper;

my $o = Foo::Bar->new;

print STDERR Dumper($o);

##</code><code>##

plxc16479> ex.pl
Foo::BUILD running
Use of uninitialized value in sqrt at Foo.pm line 17.
Foo::Bar before BUILD running
Foo::BUILD running
$VAR1 = bless( {
                 'y' => '5',
                 'x' => 25
               }, 'Foo::Bar' );