Question : Is there a way to get this filehandle without specifying a __DATA__ or __END__ section, i.e. without reopening $0 and always pointing behind the last codeline after compilation (even if there is already a __DATA__ section defined)

No, I don't think so, only stuff like open DATA, '<', \'this works too';

Otherwise if I have defined DATA filehandles in various packages, can I check which file (if existent) is associated with it?

No, I don't think so, you cannot get filename from filehandle, there is no api and no guarantee there exists a filename, but %INC ought to have package/filename associations ...

Or if there is no pure Perl way to do it, is it possible to inspect this info with help of the B backend or other magic?

I don't know but I doubt it :)

if (tmp && tmp != KEY___DATA__ && tmp != KEY___END__

http://perl5.git.perl.org/perl.git/blob?f=t/io/data.t