On my system, gpg returns 0 if there were no problems, 1 otherwise. There's no undef (or equivalent) returned so checking the return value with defined() isn't doing what you appear to think it is.

Again, on my system, both "-o file" and "--output file" are documented as "Write output to file.". Perhaps that's what you want.

Type "man gpg" at your command line to see how this is implemented on your system.