I am implying that maintaining loose coupling necessarily keeps you from being able to find certain kinds of optimizations, and that means that the top possible performance (tight memory, etc) can be hit by a tightly integrated application. If performance is a significant constraint (mini computers in the 70's, GUI interfaces on PC hardware in the 80's, PDAs in the 90s) then tightly coupled systems may be viable several years before loosely coupled ones are.

(As a practical matter, most attempts to write a tightly coupled system will result in something slower than a loosely coupled system could have been. This is certainly a strategy that one would only try with very good reason.)

This applies to a small minority of software, and applies to virtually none in the Perl world. Places where I might expect to see this happen today include extremely high-volume servers (eg Google), very constrained systems (eg many embedded projects), and computationally intensive products (some games). None of those are good candidates for Perl because Perl is relatively big and slow.