pointer to constructor to a class in perl6

To get a “reference” to .new you have to use the meta object protocol.
Either .^lookup, or .^find_method.

my $hum1-create = human1.^find_method('new');

That is still not quite what you are looking for, as methods require either a class object or an instance, as their first argument.

my $c = $hum1-create( human1 );

So you would probably want to curry the class as the first argument to the method.

my $hum1-create = human1.^find_method('new').assuming(human1);

my $c = $hum1-create();

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Note that .assuming in this case basically does the same thing as

-> |capture { human1.^find_method('new').( human1, |capture ) }

So you could just write:

my $hum1-create = -> |capture { human1.new( |capture ) }

Or if you are never going to give it an argument

my $hum1-create = -> { human1.new }

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Also you can store it in a & sigiled variable, so you can use it as if it were a normal subroutine.

my &hum1-create = human1.^find_method('new').assuming(human1);

my $c = hum1-create;

I think this is a case of an LTA error. What I understand you want to achieve, is a lambda that will create a new human1 or human2 object for you. The way you do that is not correct, and the error it causes is confusing.

my $hum1_const = -> { human1.new };
my $hum2_const = -> { human2.new };

would be a correct way of doing this. Although, I would consider this a bit of an obfuscation. Since human1 and human2 are already constants, you can assign them to a variable, and then just call new on that:

my $the_human = $condition ?? human1 !! human2;
my $c = $the_human.new;
$c.fn1;

Does that make sense?