I would put the records into groups, and then loop through the groups, each as a single entity to decide if it needs a header record. For you, the trick is preserving order so we can build the header record using data about the previous group. Luckily there's Tie::IxHash: 
use Tie::IxHash;
tie my %group, Tie::IxHash;

## group them by ID, preserving the order seen

for (@records) {
    my $id = $_->{ID};
    push @{ $group{$id} }, $_;
}

## put special header at beginning of groups larger than 1

my $last_id;
while (my ($curr_id, $records) = each %group) {
    unshift @$records, { ... } if @$records > 1;
    $last_id = $curr_id;
}

## flatten everything out

my @complete_list = map @$_, values %group;

[download]
Notice when you put the header record onto the beginning of the group, you can use $last_id, $curr_id, $records, and even $group{$last_id} to generate it.

blokhead

In reply to Re: Looping and Grouping: there must be a better way! by blokhead
in thread Looping and Grouping: there must be a better way! by edan

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