From @AlexDaniel

Code​:
my @​x; say @​x»[0]

Result​:
This type (Scalar) does not support elems
  in block <unit> at -e line 1

I think it should work (and should do nothing, as there are no elements in the array).

Here is an interesting thing. If you take, say, 2016.05 and try this​:

Code​:
my @​x; dd @​x; say @​x»[0]

Result (2016.05)​:
Array @​x = []
()

It works! But if you remove dd​:

Code​:
my @​x; say @​x»[0]

Result​:
This type (Scalar) does not support elems
  in block <unit> at /tmp/DPLGku74eh line 1

So it worked with dd, but didn't work without it. Weird. Anyway, this detail is probably not relevant anymore.

Works in a recent Rakudo:

say $*RAKU;        # Raku (6.d)
my @x; say @x»[0]; # ()