use DateTime;
subtype DateTime => as Object => where { $_->isa('DateTime') };

##</code><code>##

use DateTime;
subtype 'DateTime', as 'Object', where { $_->isa('DateTime') };

##</code><code>##

$ perl -MO=Deparse,-p -e '
use Moose;
use Moose::Util::TypeConstraints;
use DateTime;
subtype DateTime => as Object => where { $_->isa(q{DateTime}) };
'
use Moose;
use Moose::Util::TypeConstraints;
use DateTime;
use warnings;
use strict 'refs';
subtype('DateTime', as('Object', where(sub {
    $_->isa('DateTime');
}
)));
-e syntax OK

##</code><code>##

$ perl -MO=Deparse,-p -e '
use Moose;
use Moose::Util::TypeConstraints;
use DateTime;
subtype q{DateTime}, as Object => where { $_->isa(q{DateTime}) };
'
use Moose;
use Moose::Util::TypeConstraints;
use DateTime;
use warnings;
use strict 'refs';
subtype('DateTime', as('Object', where(sub {
    $_->isa('DateTime');
}
)));
-e syntax OK

##</code><code>##

$ perl -MO=Deparse,-p -e '
use Moose;
use Moose::Util::TypeConstraints;
use DateTime;
subtype DateTime, as Object => where { $_->isa(q{DateTime}) };
'
use Moose;
use Moose::Util::TypeConstraints;
use DateTime;
use warnings;
use strict 'refs';
('DateTime'->subtype, as('Object', where(sub {
    $_->isa('DateTime');
}
)));
-e syntax OK