Would any of the Perl “error variables,” $@, $!, $^E, $? (see “Error Variables” in perldoc perlvar), provide any useful information about what the command-interpreter actually saw, and/or any error messages gpg might have produced in this case?

/me nods that interpolation or the lack thereof is almost certain to be the problem here, and that separate interpolation into a string that is then executed will be the solution.