See [[bugs/Aggregated_Atom_feeds_are_double-encoded]]. By default,
XML::Atom outputs strings of UTF-8 bytes with the Perl UTF8 flag stripped
off, which IkiWiki assumes to be Latin-1 and re-encodes as UTF-8 on
output. XML::Feed does not currently (0.41-1) set the magic variable to
change this behaviour (I've filed a bug on CPAN), but IkiWiki can
usefully set the same variable as a workaround.