I've not used File::Scan::ClamAV, but I do use ClamAV for all email virus scanning. The last place I worked replaced Sophos with ClamAV for this purpose, after finding that signature updates for new viruses on ClamAV were usually as fast (and sometimes significantly faster) than Sophos. The biggest annoyance with ClamAV is that the scanning engine is frequently updated and the latest signatures will not always work with older engines, so you have to keep both the scanner and the signatures up-to-date. That being said, the signatures can be refreshed automatically (take a look at the freshclam daemon) so you really just have to worry about updating the software, and given that this is rather an exposed service you want to keep that as current as you can anyway. So this could be construed as a feature :-)

You may also want to look at using a Squid reverse proxy setup with SquidClamAV