sub binary (0|1 $n) { $n }

sub binary ($n) {
   my $k = int($n/2);
   my $b = $n % 2;
   my $E = binary($k);
   return $E ~ $b;
   }
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That's fantasy syntax, unless it was allowed in the last two weeks where I didn't have internet access. It might work along these lines:

multi binary ($n where 0|1) { $n }

multi binary ($n) {
   my $k = int($n/2);
   my $b = $n % 2;
   my $E = binary($k);
   return $E ~ $b;
}
[download]

When you post Perl 6 code these days it's worth trying it in Rakudo first, which supports all the features needed for the subroutine above (I would try it myself, but I'm currently on a machine where I can't install any programs :-/).