# REQUIRED: full article title, e.g. Perl.
# This should be unique across the data set.
my $title = $line[0] || '';

# REQUIRED: 
# A for article.
# D for disambiguation page.
# R for redirect.
my $type = $line[1] || '';

# Only for redirects, e.g. 
# an alias for a title such as
# a common misspelling or AKA.
# For example: Duck Duck Go -> DuckDuckGo.
# The format is the full title of the Redirect, e.g. DuckDuckGo.
my $redirect = $line[2] || '';

# Ignore.
my $otheruses = $line[3] || '';

# You can put the article in multiple categories, and category pages will be created automatically.
# E.g.: http://duckduckgo.com/c/Procedural_programming_languages
# You would do: Procedural programming languages\\n
# You can have several categories, separated by an escaped newline.
# Categories should generally end with a plural noun.
my $categories = $line[4] || '';

# Ignore.
my $references = $line[5] || '';

# You can reference related topics here, which get turned into links in the Zero-click Info box.
# On the perl example, e.g. Perl Data Language
# You would do: [[Perl Data Language]]
# If the link name is different, you could do [[Perl Data Language|PDL]]
my $see_also = $line[6] || '';

# Ignore.
my $further_reading = $line[7] || '';

# You can add external links that get put first when this article comes out.
# The canonical example is an official site, which looks like:
# [$url Official site]\\n
# You can have several, separated by an escaped newline though only a few will be used.
# You can also have before and after text or put multiple links in one like this.
# Before text [$url link text] after text [$url2 second link].\\n
my $external_links = $line[8] || '';

# Ignore.
my $disambiguation = $line[9] || '';

# You can reference an external image that we will download and reformat for display.
# You would do: [[Image:$url]]
my $images = $line[10] || '';

# This is the snippet info.
# It should generally be ONE readable sentence, ending in a period.
my $abstract = $line[11] || '';

# This is the full URL for the source.
# If all the URLs are relative to the main domain, 
# this can be relative to that domain.
my $source_url = $line[12] || '';

In all this may look like:

print OUT "$page\tA\t\t\t$categories\t\t$internal_links\t\t$external_links\t\t$images\t$abstract\t$relative_url\n";

# REQURIED: this is the name of the function.
my $page = $line[0] || '';

# Usually blank unless for something like JavaScript
my $namespace = $line[1] || '';

# REQUIRED: this is the target URL for more information.
my $url = $line[2] || '';

# SOME COMBO OF THESE IS REQUIRED.
# Look at https://duckduckgo.com/?q=perl+split
# The part in grey is the $synopsis and the stuff below is the $description
my $description = $line[3] || '';
my $synopsis = $line[4] || '';
my $details = $line[5] || '';

# usually blank
my $type = $line[6] || '';

# usually blank
my $lang = $line[7] || '';