I've wondered why no one upgraded Apache to 2.4 after being available for so long. Then I stumbled upon an answer on this list where it has been said that 2.4 breaks too many packages .
I've taken some time to verify what does it break, and fixed all packages from [extra] and [community] which had direct dependency on the "apache" package (mod_whatever ones).
The mod_fcgid, mod_wsgi2 and mod_wsgi just need simple rebuilds against newer versions.
PHP case was somehow strange. I had to modify some configure switches. Without --disable-cgi I'd get undefined references at build time and without --with-apsx=/usr/bin/apsx I'd get undefined references at run time.
For other ones, I've found patches online and assembled them for direct applying in Archlinux svn (I used svn add and svn diff)
In case someone is interested, patches can be found at . It's free hosting, so it might "suck" sometimes.
I've tested the packages against apache24 package from AUR. By tested, I mean compiled, loaded into httpd and checked that httpd doesn't fail to start.
As a side note, apache24 uses event_mpm by default, but you might need to change to prefork one to use PHP module. Also, with apache 2.4 you can use php-fpm via mod_proxy as described at 
As for the perl module, I had to use svn checkout of the httpd24 branch. Patch was getting way to big to ship in package.