A few things:

1. "shared_clone" returns a new object, and your code does not store that value anywhere.
2. BUILD does not change the value returned by 'new'. I think you need to use an "around" modifier for "new".
3. I don't see any reason bar would have a value except that you must have left some code out of the example somewhere.

To sum up:

use threads;
use strict;

{  

    package ThreadableObj;
    use Moose;
    use threads::shared;
    use Data::Dumper;

    around 'new' => sub {
        my $orig = shift;
        my $class = shift;
        my $self = $class->$orig(@_);
        my $shared_self : shared = shared_clone($self);

        # here the blessed() already be the version in threads::shared
        print Dumper($shared_self),"\n";
        return $shared_self;
    };

    has foo => ( is => 'rw', isa => 'Str' );
    has bar => ( is => 'rw', isa => 'Int' );
}

# here lives
my $obj : shared = ThreadableObj->new(foo=>'foovalue');
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