On December 24 each year since the beginning of time (ok, since about 280 AD), Saint Nicholas of Myra (aka Santa Claus) has travelled the world, depositing lollipops in the stockings of nice children and for the naughty ones, a lump of coal.

His age aside, did you ever wonder how he manages to do it? The answer is Alabaster "Ali" Snowball - Elf Administrator of the nice-and-naughty list. But what does he actually do?

The Night Before Christmas

Just like your parents did while trying to get you to sleep, I'll let you follow this fairy tale with the happy ending. Ali has a list like:

my @neighbourhood = (
    {
        name      => 'Donald',
        age       => 3,
        behaviour => 'naughty',
        stocking  => [],
    },
    {
        name      => 'Theresa',
        age       => 5,
        behaviour => 'nice',
        stocking  => [],
    },
    {
        name      => 'Angela',
        age       => 17,
        behaviour => 'nice',
        stocking  => [],
    },
    ...
);

Then he goes through like this

foreach my $person (@neighbourhood) {
    next unless $person->{age} < 16;
    if ($person->{behaviour} eq 'nice') {
        push @{$person->{stocking}}, 'lollipop';
    }
    else {
        push @{$person->{stocking}}, 'coal';
    }
}

Or, if his code is as terse as he is in conversation it's more likely to be:

map {
    push @{$_->{stocking}},
        $_->{behaviour} eq 'nice' ? 'lollipop' : 'coal';
}
grep { $_->{age} < 16 } @neighbourhood;

From which we get a neighbourhood of

(
    {
        name      => 'Donald',
        age       => 3,
        behaviour => 'naughty',
        stocking  => ['coal'],
    },
    {
        name      => 'Theresa',
        age       => 5,
        behaviour => 'nice',
        stocking  => ['lollipop'],
    },
    {
        name      => 'Angela',
        age       => 17,
        behaviour => 'nice',
        stocking  => [],
    },
    ...
)

and everyone lives happily ever after (especially Donald).

The Recursive Falling Dream

Alas, like all good children on the list, you're rather naive. You see, even in 280AD Myra was a boomtown without a simple linear list of properties, and dealing with the whole world in modern times it's an even more complex data structure with snippets like:

my $world = [
    {
        name        => 'USA',
        type        => 'Country',
        territories => [
            ...
            {
                name    => 'Washington, D.C',
                type    => 'District',
                streets => [
                    {
                        type       => 'Avenue',
                        name       => 'Pennsylvania',
                        properties => [
                            {
                                type        => 'House',
                                colour      => 'White',
                                inhabitants => [
                                    {
                                        name      => 'Donald',
                                        age       => 3,
                                        behaviour => 'naughty',
                                        stocking  => [],
                                    },
                                    ...
                                ],
                            },
                        ],
                    },
                ],
            },
        ],
    },
];

Scary, isn't it? Do you have that queasy, roller-coaster-after-Christmas-pudding feeling?

If you need to traverse deeply nested data where you're not sure of the structure don't reinvent the wheel - Data::Rmap has done it for you

use Data::Rmap qw/rmap_hash/;

rmap_hash {
    push(@{$_->{stocking}}, what_they_deserve($_))
        if they_deserve_anything_at_all($_)
} $world;

In that snippet rmap_hash recursively traverses the data structure and applies the code block to every hash reference it finds. Then it's simply a matter of implemeting these two validation methods:

sub they_deserve_anything_at_all {
    my $person = shift;

    foreach my $key (qw/name age behaviour stocking/) {
        return unless exists($person->{$key});
    }
    return unless ref($person->{stocking}) eq 'ARRAY';
    return unless $person->{age} < 16;
    return 1;
}

sub what_they_deserve {
    my $person = shift;

    return $person->{behaviour} eq 'nice' ? 'lollipop' : 'coal';
}

Avoiding the Trap

The problem is that the world is more than just a bunch of countries. Apart from countries there are other entities like:

{
    name  => 'Jungle',
    type  => 'Company',
    staff => [
        {
            name      => 'Jeff',
            age       => 7,
            behaviour => 'naughty',
            stocking  => [],
            role      => 'CEO',
        }
    ],
}

and

{
    name    => 'Monaco',
    type    => 'Tax Haven',
    evaders => [
        {
            name      => 'Ebenezer "Bono" Scrooge',
            age       => 12,
            behaviour => 'nice',
            stocking  => [],
            role      => 'Philanthropist',
        }
    ],
}

In this case, rather than wasting treats or Ali's time, we ignore these branches with Data::Rmap's cut method.

rmap_hash {
    Data::Rmap::cut if $_->{type} && $_->{type} =~ m{^(Company|Tax Haven)$};

    push(@{$_->{stocking}}, what_they_deserve($_))
        if they_deserve_anything_at_all($_)
} $world;

...and a Happy New Year

As you can see, Santa's performance has been improved by skipping large portions of the world, but it's still a CPU intensive trip. Don't recurse through your data if you can avoid it.

If you can't avoid recursion though, don't reinvent the wheel. Let Data::Rmap do the traversal so you can just focus on the reasoning.