Not to defend this particular programming idiom, but the third result for [perl ...

pfedor on Oct 4, 2009 | parent | context | favorite | on: Perl 5.11 released

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 so it's rather easy to Google.