MyObject.pm

package MyObject;
use Moose;
use Moose::Util::TypeConstraints;
use MooseX::Types::DateTime;
use DateTime::Format::ISO8601;
use namespace::autoclean;

subtype 'DateTime8601',
    as 'MooseX::Types::DateTime';

use Data::Dumper;
coerce 'DateTime8601',
    from 'Str',
    via {
        $_ = DateTime::Format::ISO8601->parse_datetime($_);
    };

has 'CreationDate' => ( is => 'ro', isa => 'DateTime8601', coerce => 1 );

__PACKAGE__->meta->make_immutable;
1;

myscript.pl

#!/usr/bin/perl

use strict;
use warnings;

use MyObject;
use Data::Dumper;

my $x = MyObject->new(CreationDate => '2011-09-02T18:22:05.526Z');
print Dumper($x);

output.log

Attribute (CreationDate) does not pass the type constraint because: Validation failed for 'DateTime8601' with value DateTime=HASH(0x25cc9f0) at constructor MyObject::new (defined at MyObject.pm line 20) line 35
        MyObject::new('MyObject', 'CreationDate', '2011-09-02T18:22:05.526Z') called at ./myscript.pl line 9