The cost of a new webapp machine (including all provisioning, bandwidth, sysadmin costs, etc) is generally the cost of 1 week of developer time. The cost of a new database machine (about 4x as powerful) is generally 2 weeks of developer time. If I can write something in 1 week (in Perl) and it requires 2 machines or take 4 weeks (in C++) and it requires 1, I have saved two weeks of my salary. This assumes that the C++ version even does everything the Perl one does and that the C++ one requires half the resources of the Perl one. (Both have proven to be faulty assumptions).

Furthermore, the Perl version is going to be easier and safer to extend than the C++ version. New developers are going to be productive more quickly. And, there is more battle-tested code for Perl (via CPAN) than there is for C++.

The key is determining where your costs are. 30 years ago, the significant cost was hardware, so you optimized for hardware speed. Today, the significant cost is developer time, so you optimize for developer speed. This is the origin of "throwing hardware" at a problem. It's usually the right business decision.