I like to use early exits because they often save a lot of obfuscating indentation and state management. Test simple cases first and exit early. Complicated cases then tend not to be indented and the conditions that get you to the complicated case are generally much easier to see. Note that this early exit technique works in the context of loops just as well as it does for subs.

To me a single point of exit seems a rather artificial constraint that often requires extra testing and nesting to achieve. Removing that constraint very often allows much clearer logic flow - it's easier for people to understant and prove, which is much more important than making it easier for compilers.

Using undef as the failure case allows 0 to be used as a valid return value. That was part of what I was alluding to in the paragraph about "sanity checking".

C required that all variables be declared at the start of a block. C++ and Perl (to name but two) allow variables to be declared wherever they are needed. So yes, I did mean to take advantage of that feature of Perl (and other languages) to declare variables as close as possible to their first use.