package Foo::Bar;

use Moose;
extends 'Foo';

use Data::Dumper;

around 'BUILDARGS' => sub {
    my ($orig, $class, %args) = @_;
    print STDERR "Foo::Bar::BUILDARGS running.\n";

    $args{x} = 25;

    return $class->$orig(%args);
};

1;

##</code><code>##

plxc16479> ex.pl
Foo::Bar::BUILDARGS running.
Foo::BUILD running
$VAR1 = bless( {
                 'y' => '5',
                 'x' => 25
               }, 'Foo::Bar' );