How about instead of () you insert die("Deferred implementation can't be executed");.

Perhaps, but when I used it I just wanted a placeholder, not necessarily an abstract op (yes, I made that up ;) and I also have no idea how it will be implemented in perl6 (it may be described in p6l ... somewhere). Maybe we need YAO instead

package bangbangbang;

use Filter::Simple;

FILTER_ONLY
  code => sub {
    s{ \s \Q!!!\E \s }
     { die("Deferred implementation can't be executed"); }gx
  };

q[ ichi ni san ];
[download]

And obligatory example usage

use bangbangbang;

sub foo { !!! }
my $bar = sub { !!! };
sub baz { return !!! }

eval { foo()   } or warn "foo() - $@";
eval { &$bar() } or warn "bar() - $@";
eval { baz()   } or warn "baz() - $@";

__output__

foo() - Deferred implementation can't be executed at bbb_test.pl line 
+3.
bar() - Deferred implementation can't be executed at bbb_test.pl line 
+4.
baz() - Deferred implementation can't be executed at bbb_test.pl line 
+5.
[download]

HTH

_________
broquaint