ENV and BASH_ENV give the names of files that are executed when a new shell is started. Someone could point one of those to a file full of evil commands.

CDPATH gives a search path for the cd command. Changing that could make shell scripts switch into unexpected directories.

IFS gives a list of "internal field separators", which the shell treats like spaces. Here's a link that tells how to exploit this feature.

Yeah, they have to do with the shell. If you call any external programs, can you be absolutely certain that none of them will invoke a shell?

I doubt that this is a complete list of problem environment variables. It's probably best to delete everything you aren't sure is safe.

%ENV = map { ($_, $ENV{$_}) }
       grep exists($ENV{$_}), qw(THIS THAT THE_OTHER);
[download]

...or something of that sort. Anyone have any suggestions of environment variables that will cause problems if they aren't kept? TERM? USER? HOME?