Linux only initially copies the page tables (very small). Solaris, on the other hand, does copy all of the address space (unless you use vfork).

Thanks for the correction. I didn't know that some systems (i.e. Solaris) didn't use copy-on-write.

In both environments, text (ie code) pages are read-only.

And thus they can't possibly be used. Perl wouldn't be able to populate it if it started read-only. Additionally, it can't become read-only (if that's possible) because Perl can compile more code at any time (as mentioned earlier).