I've upgraded server running OpenWebmail from Debian 4.0 to Debian 5.0. Perl was upgraded to 5.10 as well. I have issues with some functions of OpenWebmail (attachments are 0 bytes size, HTML-formatted e-mails are broken), may be there is some patch for Openwebmail or compatibility mode for Perl?
ask on the OpenWebMail mailing list, and include your OpenWebMail version number:
http://openwebmail.acatysmoof.com/archive