Some minor corrections:

The first time he did
   undef $/;
so that he could read in all of <DATA> at once, and then he set
   $\=$:;
(note the different lean of the second character) which is the output record seperator so that his output is formatted properly. As a note, he sets it to " \n-" (the format line break variable) and then removes the hyphen, so his output record seperator is now a space followed by a "\n". Very convenient ;)

He formats $_ to have no line breaks or white space, then the map takes care of using the DATA as logic for how to visually represent the material. Since it's a map, when he prints $a it'll get treated as part of an array, which is useful for putting in the "\n" without being seen (from the output record seperator). Finding out the logic of the DATA section is left as an exercise for the reader.

Hope That Helps,
jynx

update: reworded a few lines...