Sharing read/WRITE data between threads?

I am using Apache/2.4.41 (Ubuntu), with mod_perl.  Apache uses multiple 
threads, and I want to share read/WRITE data between threads.  (I.e., I 
want to be able to modify some shared data in one thread, such that 
other threads can see those changes.)  In "Practical mod_perl" Stas 
Bekman describes how to share read-only data between threads, but says 
nothing about how to share WRITABLE data between threads.

Any clues about how this can be done?  I've searched high and low and 
found nothing.  I will also want to know what mechanisms are available 
to coordinate access to that shared data, such as locks or semaphores.

I also posted this message to StackOverflow, but got no response so far:
https://stackoverflow.com/questions/68901260/how-to-share-read-write-data-in-mod-perl-apache2

Any help would be appreciated!

Thanks,
David Booth