Not to defend this particular programming idiom, but the third result for [perl underscore] is "About Special Underscore Filehandle" http://www.perlmonks.org/in

Not to defend this particular programming idiom, but the third result for [perl underscore] is "About Special Underscore Filehandle" http://www.perlmonks.org/index.pl?node_id=613621 http://www.perlmonks.org/index.pl?node_id=613621 so it's rather easy to Google.