Believe me...   the best way to approach “child processes|threads” is as though they were the now-college-age kids that you get a postcard from once a month IM from every five seconds and that you try your best not to seriously worry about in the meantime.

Once you launch a child process or child thread to do some particular task, as a very-basic design principle do not seek to meddle in its affairs.   Do not design nor contemplate anything that is to the very slightest degree dependent upon any artifact whatsoever of “timing,” for as surely as such silly notions may (seem to) prove themselves in testing, forsooth ... they shall surely prove a fool of ye in actual production!

Heh™ ...   “Abandon all hope, ye who enter here ...”

P.S.:   The aforesaid admonition has nothing whatsoever in-particular to do with “Perl in-particular.”   It is a VNIVERSAL TRVTH ...