http://fabdp.fh-potsdam.de/lv-neu/online-courses/perltutorial/perl_start.html
http://www.comp.leeds.ac.uk/Perl/start.html
http://perldoc.perl.org/perldata.html
http://www.ayni.com/perldoc/perl5.8.0/pod/perlthrtut.html#Shared-And-Unshared-Data
