Recursive quickshuffle

quickshuffle.pl6

sub shuffle( @arr is copy ) {
  my sub qshuf ( $first is copy, $cnt is copy ) {
    loop {
      return if $cnt <= 1;
      my ($left, $right) = ($first, $first+$cnt);
      SWAPLOOP: loop {
        ++$left >= $right and last SWAPLOOP while Bool.pick;
        --$right <= $left and last SWAPLOOP while Bool.pick; 
        @arr[$left++, --$right] .= reverse;
        last if $left >= $right;
      }
      my $lsize = $left - $first;
      my $rsize = $cnt - $lsize;
      if $lsize < $rsize {
        qshuf( $first, $lsize );
        ($first, $cnt) = ( $left, $rsize );
      } else {
        qshuf( $left, $rsize );
        $cnt = $lsize;
      }
    }
  }
  qshuf( 0, +@arr );
  return @arr;
}

say shuffle( flat ^30 );