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;