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sub fibonacci {
   my ($n) = @_;
...
   return 1 if $n == 1;
   return sum fibonacci($n-2), fibonacci($n-1);
}

sub fibonacci {
   my ($n) = @_;
...
      async { fibonacci($n-2) },
      async { fibonacci($n-1) };
}

sub fibonacci {
   my ($n) = @_;
...
      async_maybe { fibonacci($n-2) },
      async_maybe { fibonacci($n-1) };
}

sub fibonacci {
   my ($n) = @_;
...
   #      must call $on_complete->($x+$y).

}