Basically, __DATA__ provides a quickie way to bundle data for a script right there at the bottom of the script, and make it available to you via the filehandle <DATA>, rather than have to (lazy-speak) put the data in a text file and make sure you always have access to that file, open it, etc. All text after a line containing '__DATA__ will be ignored in terms of executable code. __END__ functions similarly. Usually (?)this is for including test data for your script.

In looking for online documentation about __DATA__, the best I could find was contained in a couple of paragraphs from perldata.html (using ActiveState) which in turn said "See the SelfLoader manpage for more description of __DATA__, and an example of its use." Not that said description was crystal clear... Good Luck! :-)