This is a simplified version of Dan Deal's answer, with a few notes for less experienced developers.
You'll need to install Xcode 6.1 from the Mac App Store. Xcode is suite of tools developed by Apple for developing iOS and OS X software. It takes up almost 6gb but can be deleted after this installation.
Start Xcode once to agree to Apple's terms.
In Terminal, change to any temporary directory then download mod_perl 2.0.9-dev:
(Caution - the 'any temporary directory' must be on your root volume and must not have
any space characters in the directory name; otherwise the make scripts will fail later on)
svn checkout https://svn.apache.org/repos/asf/perl/modperl/trunk/ mod_perl-2.0
Change to the newly created mod_perl directory:
cd mod_perl-2.0
Tell the installer where to look for parts:
/usr/bin/apr-1-config --includedir /usr/include/apr-1
sudo ln -s /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk/usr/include/apache2 /usr/include/apache2
sudo ln -s /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk/usr/include/apr-1 /usr/include/apr-1
(ln -s makes a symbolic link, and the apr-1-config program is used to retrieve information about the apr library, and is typically used to compile and link against the library.)
(Caution - on some Yosemite installations the /usr/include directory does not exist; you
may have to create it by cd /usr;mkdir include)
Make mod_perl:
perl Makefile.PL MP_CCOPTS=-std=gnu89; make ; sudo make install
Delete the temporary folder mod_perl-2.0.
Tell apache to include mod_perl in the apache httpd.conf:
sudo vi /etc/apache2/httpd.conf (or)
sudo nano /etc/apache2/httpd.conf
Add the following line at the end of the list of includes, near line 170:
LoadModule perl_module libexec/apache2/mod_perl.so
Save, quit, and restart apache:
sudo apachectl restart