IMO the module should explain in the POD, in what cases using $^X is not enough or how it is different from Probe::Perl.
Anyway, quoting CPAN Testers' CPAN Authors FAQ, $^X is not enough when:
1) perl was executed with a relative path and the script has chdir()ed; 2) because $^X originates in C's argv (in the main() function) it is possible for the calling program to exec() in such a way that argv isn't the path to the interpreter; 3) HP/UX can do weird stuff in scripts that use #!; 4) VMS. (Not clear about #4 though :) ).