Here is the official Perl5 todo list: http://perl5.git.perl.org/perl.git/blob/HEAD:/Porting/todo.pod
You could also:
* Compile Perl5 blead and install modules (http://perlbrew.pl - `perlbrew install blead; perlbrew use perl-blead`)
* Look on CPAN for modules that have test failures on blead, then work on fixes
* Learn to use perlbrew, cpanm, and other parts of the modern Perl toolchain (http://perlbrew.pl & http://cpanmin.us ... see also http://www.slideshare.net/alex.muntada/modern-perl-toolchain )
* Have a look at the London PM github repository - https://github.com/LondonPM
* Play with the code for dipsy, the #london.pm resident infobot
* If Christian is around, get functional-perl (https://github.com/pflanze/functional-perl) explained to you and tell whatever you think, write examples, etc.
* Want a better wiki? This is an installation of Quiki, which is a simple wiki from the CPAN. You can play with it, or modify the CSS, or ?
Feel free to follow up with other hacking topics.