useful technique for initializing something with a reference
From Rainer Weikusat@21:1/5 to All on Mon Nov 1 18:44:15 2021
Minor gem of Perl's support for a conditional assignment operators.
Assuming that $x holds a reference to some container and a slot/ key
of it is supposed to refer to another container which is initially undef
and a variable made $y is supposed to be assigned this reference for
easier subsequent access, this can be set up with the single line
$y = $$x[0] //= {};
If the value of $$x[0] is defined, ie, has already been initialized,
it'll be assigned to $y. Otherwise a reference to an anonymous hash is
assigned to $$x[0] and then to $y.