With proper scoping of variables and avoidance of circular data structures, this should never happen. I'm willing to bet you are doing at least one of those.

Your statement that you have to undef your hashes, to me, says you're not properly scoping your variables. If you properly scope your variables to the tightest-scope, Perl will do all the garbage collection for you.

Real-life circular data structures are most easily demonstrated by the following:

my %child = (
    a => 1,
);
my %parent = (
    child => \%child,
);
$child{parent} = \%parent;
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Now, unless you explicitly break the circular reference (or use WeakRef), the memory used by %child and %parent will stick around until the process is done, even if the variables are out of scope.