Fixed: Perl 5.18.1 problems with MIME::Entity (bug#9832).

See:
http://search.cpan.org/~rjbs/perl-5.18.0/pod/perldiag.pod#Strings_with_code_points_over_0xFF_may_not_be_mapped_into_in-memory_file_handles
http://blogs.perl.org/users/tony_cook/2013/02/perl-io-on-scalars.html

Thanks to Niko Tyni at the Debian team for investigating!