You'll notice that I am trying to explicitly merge stdout & stderr. The problem is that backticks ignore my explicit direction.

Uhm, you *are* capturing both STDOUT and STDERR. Other than that your test program is written in such a way that it neither proves, nor disproves that you are capturing both, what makes you think it doesn't?

Hint, change your print "Captured $x" in print "Captured --->$x<---\n" and it should be obvious.