So, here goes.

Yes, we can piggyback the dynamic variables to detect whether we’re inside of the block. This short snippet illustrates the principle:

sub outer(&block) {
  my @*piles;
  &block()
};

sub inner {
  say @*piles;
  CATCH {
    when /Dynamic/ {
      die "You can only use &inner in &outer"
    }
  }
};

inner() # boom

Using the dynamicals to detect whether we’re inside of the outer block carries with it the risk of false positives (i.e. someone else declares a dynamical with the same name, causing the inner sub to think it’s in the outer when it isn’t). But this risk is inherent in this use of dynamicals, not in the error munging.