I get this warning too, but I'm using an old version of CGI (3.49). The POD for my version does not mention the $CGI::LIST_CONTEXT_WARN variable. However, the latest version on CPAN (4.21) does mention this variable. If you want to use the variable and you have an old version, you need to update. Note that, according to perl5220delta, CGI was removed from Core.