STDOUT or STDERR should neither one return a numeric 0. They should only return a string (although the string could be '0' if that's what the program you executed wrote to one of those handles).

If you're getting a numeric 0, you're getting it from a system call or a pipe. In scalar context the backticks will return either a string or undef. perlop says so. In a list context, it will be a list of lines or an empty list. Again, perlop says so.