sub divide {
  my $mid = @_/2;
  $ITER++;
  return if @_ == 1;
  divide(@_[0 .. $mid - 1]);
  divide(@_[$mid .. $#_]);
}

for (10 .. 30) {
  $ITER = 0;
  divide(1 .. $_);
  printf "%2d  %2d\n", $_, $ITER;
}