You need to wrap your code in <code> and </code> tags. The characters [ and ] are special to perl monks and can't be put as literal text in your post. You should also use <p> and </p> tags around paragraphs.

At a guess you have cut and pasted this code from the documentation

Env::C::setenv($key, $value, [$override]);
[download]

where [$override] is intended to be read as an optional parameter.

If you fix your post with code tags and an explanation of what your are trying to achieve you will get a better response. Please read How do I post a question effectively?.