hence no need to be copied.

No, there is a need: For Perl to see them.

The buffers of Perl strings are subject to modification and freeing at any time. It *might* be possible to tell Perl the PV associated with a scalar can't be changed, but

• there's no way to tell Perl to use a existing memory block for the string buffer of a string, and
• there's no way to tell Perl the string buffer of a string can't be freed.