* http://www.perlboard.de/perlguide/Kap06.html
  * http://www.ssw.uni-linz.ac.at/Teaching/Lectures/Sem/2000/Katzmayr/tutorial/kontroll.html
  * http://www.tutorialspoint.com/perl/perl_loops.htm
  * http://perldoc.perl.org/perlsyn.html