pandoc.hs

    , Option "V" ["variable"]
                 (ReqArg
                  (\arg opt -> do
                     let (key,val) = case break (`elem` ":=") arg of
                                       (k,_:v) -> (k,v)
                                       (k,_)   -> (k,"true")
                     return opt{ optVariables = (key,val) : optVariables opt })
                  "KEY[:VALUE]")
                 ""

I want to pass this a key with multiple variables. How can I do that? I can see that if no value is provided it defaults to true, but what does the line above that do?

-V KEY[=VAL], --variable=KEY[:VAL]
    Set the template variable KEY to the value VAL when rendering the document in standalone mode. 
This is generally only useful when the --template option is used to specify a custom template, since 
pandoc automatically sets the variables used in the default templates. If no VAL is specified, the key will 
be given the value true.

See http://johnmacfarlane.net/pandoc/README.html

Their templates look like this:

$for(author)$
<h2 class="author">$author$</h2>
$endfor$

Custom delimiters can be defined as follows:

$for(tags)$tags$$sep$, $endfor$

See http://stackoverflow.com/questions/25559469

There's currently no way to set complex values like this from the
command line.  You'd have to embed them in the source file as YAML, or
in a separate file that you include on the command line. 

See https://groups.google.com/d/topic/pandoc-discuss/r-t0hXKSMTM/discussion