perl -Mthreads -Mthreads::shared -Mstrict 
-wle"
    my $c:shared; 
    async{ 
        ## This next line triggers the "Invalid value for shared scalar"
        $c = sub{ print 'Hi'}; 
        sleep 100
    }->detach;; 
    sleep 3;
    $c->();
    sleep 100"

Thread 1 terminated abnormally: Invalid value for shared scalar at -e line 1.
Can't use an undefined value as a subroutine reference at -e line 1.