I have a list of items to do something to; and an index that tells me which one to fiddle with next. This index is incremented after each twiddle; and wraps to the start at the end of the list. This is simple to code: 
my @items = (1,5,6,3,5,2,4);
my $index = 0;

sub do_next
{
  do_something($items[$index]);
  $index = ($index+1) % @items;
}

[download]
But now we need the ability to vary our list of items --Dave
my @items = (1,5,6,3,5,2,4);
my $index = -1;

sub add_item
{
  push @items, @_;
}

sub do_next
{
  if (@items)
  {
    $index = ($index+1) % @items;
    do_something($items[$index]);
  }
  else
  {
    $index = -1;
  }
}

[download]
To understand the additional complexity in do_next, consider the case when: we hit the last item in the list; then add an item; then do_next() again.

Now the last part: deleting items while maintaining the index. The interesting facts are that the list may have duplicates; and if we delete an item that is earlier in the list than the current index, then we must decrement $index. Here's the code: 

sub remove_item
{
  my %del = map {$_=>1} @_;

  my @part_1 =
    grep { !exists $del{$_} } @items[0..$index];
  my @part_2 =
    grep { !exists $del{$_} } @items[$index+1..$#items];

  $index = $#part_1;
  @items = (@part_1, @part_2);
}

[download]

In reply to round-robin on varying sequence by dpuu

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