Thanks choroba, I didn't know about those Role::Tiny methods.

I am not sure about whether I prefer them to eval "with '$module'; 1;", though.

I had to load Role::Tiny explicitly not importing anything, since its before(), after(), around() and with() methods clash with Moo's :

Subroutine MyClass::before redefined at [...]/5.22.0/Role/Tiny.pm line
+ 58.
Subroutine MyClass::after redefined at [...]/5.22.0/Role/Tiny.pm line 
+58.
Subroutine MyClass::around redefined at [...]/5.22.0/Role/Tiny.pm line
+ 58.
Subroutine MyClass::with redefined at [...]/5.22.0/Role/Tiny.pm line 6
+7.
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And even when I do that I can't use apply_roles_to_object() inside sub BUILD, because the parent's methods haven't been loaded and thus the child's will be.

package MyClass::Child;

use Moo::Role;

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

1;
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package MyClass;

use Moo;
use Role::Tiny();

sub BUILD {
    my $self = shift;

    Role::Tiny->apply_roles_to_object( $self, 'MyClass::Child' );

    return $self;
};

sub foo { return 'foo'; }

1;
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Output:

$ perl -MMyClass -E '$o = MyClass->new; say $o->foo, $o->bar;'
horribly at MyClass/Child.pm line 5.
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So the only way I could make it work as part of the Moo constructor was with apply_roles_to_package() :

package MyClass;

use Moo;
use Role::Tiny();

sub BUILD {
    my $self = shift;

    Role::Tiny->apply_roles_to_package( __PACKAGE__, 'MyClass::Child' 
+);

    return $self;
};

sub foo { return 'foo'; }

1;
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Output:

$ perl -MMyClass -E '$o = MyClass->new; say $o->foo, $o->bar;'
foobar
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Thanks again for the pointer.