package MyClass;

use Moo;
with 'MyClass::Child';

sub foo { return 'foo'; }

1;

##</code><code>##

package MyClass::Child;

use Moo::Role;

sub foo { die 'horribly'; }
sub bar { return 'bar'; }

1;

##</code><code>##

#!/usr/bin/perl
use strict; use warnings; use feature 'say';
use MyClass;

my $obj = MyClass->new;
say $obj->foo;
say $obj->bar;

__END__

##</code><code>##

$ perl 1158279.pl
foo
bar

##</code><code>##

package MyClass;
use Moo;

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

    my %modules = (
        1 => 'MyClass::MyChild',
        0 => 'Acme::Frobnicate',
    );

    my $module = $modules{ $args->{'be_serious'} };
    eval "with '$module'; 1;" or die $@;

    return $self;
}           

sub foo { return 'foo'; }

1;