None of the Moose built-in types treat overloaded objects as the non-object types they overload.

So for example, an object overloading "" will not pass the Str type constraint. And objects overloading 0+ will not pass Num or Int.

Whether you think this is a good design decision or not, it's the way Moose works, and changing it now would likely break a lot of stuff that relies on the existing behaviour.

Luckily, you don't need to use the built-in Moose Bool type. You can define your own type constraints.

Moose::Util::TypeConstraints makes this possible. MooseX::Types makes it a bit easier. But (and I'm biased on this!), I'd recommend using Types::Standard which is a type constraint library that works with Moose, Mouse, and Moo, but doesn't require any of them. Here's one way you could achieve your aims with it:

{
    package My::Package;
    use Moose;
    use Types::Standard qw( Bool Overload );

    has attr => (
        is     => 'ro',
        isa    => Bool | Overload["bool"],
    );

    __PACKAGE__->meta->make_immutable;
}
[download]

Or even:

{
    package My::Package;
    use Moose;
    use Types::Standard qw( Bool Overload );

    has attr => (
        is     => 'ro',
        isa    => Bool->plus_coercions( Overload["bool"], sub { !!$_  
+} ),
        coerce => 1,
    );

    __PACKAGE__->meta->make_immutable;
}
[download]

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