This [...] only works if undef is not a possible value, and empty arrays are not possible.

It doesn't even handle false values (without a minor change).

The following handles false values (including undef) as well as empty arrays:

sub genIterator {
  my $array = shift;
  my $index = 0;   # Current index into $array
  my $child_iter;  # Iterator for array at $array[$index]
  return sub { 
    while (1) {
      # If we're iterating through descendants of a child,
      if (defined $child_iter) {
        # If the there is a descendant we haven't returned,
        if ( my ($element) = $child->() ) {
          return $element;
        }

        # No more descendants for this child.
        $child_iter = undef;
      }

      # Signal end of iteration if done.
      return if $index >= @$array;

      # Return next child if it's not an array ref.
      my $element = $array->[$index++];
      return $element if ref $element ne 'ARRAY';

      # Prepare to iterate through descendants of child.
      $child_iter = genIterator($element);
    }
  }
}

my $iter = genIterator \@nested;
say while ($_) = $iter->();
[download]