(If you use slices with just one element you will get a warning though.)

That depends on how you write the slice:

@a = (1 .. 5);
say @a[1];     # Warning
say @a[1..1];  # No warning
say @a[1,];    # No warning
@b = 1;
say @a[@b];    # No warning
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Four one element slices, but only one warns.

Frankly, I find the warning a bit silly; specially the warning if the slice is used in rvalue context. There isn't anything else the programmer could have reasonably meant. And in Perl6, @a[1] is going to be the required syntax anyway.