When spawning a worker, the boss does a fork, immediately followed by an exec, which is much more memory friendly (on modern Linux/Unix systems, copy-on-write means you need barely extra memory for a fork when neither of the programs modify the data).

copy-on-write does indeed make a fork more efficient, but you lose all that efficiency the minute you do an exec because the exec'd process starts over from scratch, loading its own separate copy of the perl interpreter, its own separate copy of the script, all the modules, etc. -- all taking up memory which would otherwise have been shared with the parent. This makes it LESS memory-friendly than just forking.