sub linear_sequence {
  my ($start, $inc) = @_;
  sub {
    my $value = $start;
    $start += $inc;
    return $value;
  };
}

sub interleave_linear_sequences {
  my @gen = map linear_sequence(@$_), @_;
  sub {
    push @gen, shift @gen;
    $gen[-1]->();
  }
}

sub alternating {
  interleave_linear_sequences([1, 2], [-2, -2]);
}

sub rearranged {
  interleave_linear_sequences([1, 4], [3, 4], [-2, -2]);
}

my $a = alternating();
my $r = rearranged();

my %do_report = map {$_=>1} qw(10 100 1000 10000);

my $count = 0;
my $alternating_sum = my $rearranged_sum = 0;
while (%do_report) {
  $count++;
  $alternating_sum += 1/$a->();
  $rearranged_sum  += 1/$r->();
  if (delete $do_report{$count}) {

    print "$count terms:\n",
          "  Alternating: $alternating_sum\n",
          "  Rearranged:  $rearranged_sum\n";
  }
}