Hi,
for now I can only give you these references: http://thread.gmane.org/gmane.org.wikimedia.mediawiki/4024/focus=4100 from the archive. and from the apache (here version 1.3, but you'll find the version concerning your apache) http://httpd.apache.org/docs/1.3/misc/FAQ-F.html#premature-script-headers
Has your php gone cgi-php during the week end ? or changed directory. Can Perl be a problem in the cgi's reading?
Sorry.
François