The use of system, exec, qx, backtick, string eval, open with pipe, unlink.

I am sure I missed a few. Any other expression that you think might be dangerous?

truncate, syscall, sysopen, fork, kill

There are various levels of paranoia you can apply, with more paranoia meaning more work. For example you can set up a virtual test machine with an selinux enabled kernel (or openbsd, which is paranoid by default, or a solaris zone, which is also said to be quite secure), where you can do things like forbid network access, prevent writing or even reading files with different security context etc.

But I know of no solution that's both secure and little effort.