I suppose, if you want to be formal about it.
iirc, __END__ and __DATA__ are equivalent, and that's why the script runs. Please correct me if you know otherwise.
Running a script - perl script.pl - the tokens make no difference. But if you invoke perl on the command line to read STDIN and paste the script, __END__ makes DATA into STDIN, and you have to type Ctrl-d to close STDIN/DATA. Not a big deal, but that's the difference ;-)