Is that one argument or none for a Perl 6 block?

As far as I know, the only way to know the number of parameters passed without an explicit signature, is to use @_ inside the body, which will generate a :(*@_) signature.

my &block := { say "Got @_.elems() parameter(s)" };
block;               # Got 0 parameter(s)
block 42;            # Got 1 parameter(s)
dd block.signature;  # :(*@_)

Yeah, the good old @_ is still there, if you want it :-)

{ put $_.perl }

Is sort of similar to this: (which doesn't work)

-> ;; $_? is raw = CALLERS::<$_> { put $_.perl }

Since the default is default for $_ outside of the block is Any, if you don't place anything into $_ before you call the function you get Any.

---

To get something at all similar where you can tell the difference use a Capture :

my &foo = -> ;; |C ($_? is raw) {
    unless C.elems {
       # pretend it was defined like the first Block above
       CALLER::<$_> := CALLER::CALLERS::<$_>
    }
    my $called-with-arguments := C.elems.Bool;


    if $called-with-arguments {
        say 'called with arguments';
    } else {
        say 'not called with arguments';
    }
}