Hey there,
just updated to 3.5 (no Errors on Update) - now Cron was running - and I got Error Mail:
Cron apache@stats /usr/bin/php /var/www/piwik/console core:archive --url=https://stats.futureweb.at/ > /var/log/piwik.cron.log
PHP Fatal error: Cannot use result of built-in function in write context in /var/www/piwik/vendor/pear/archive_tar/Archive/Tar.php on line 662