Perl has a "leak" with circular references. I sort of boggled at your code for a while since it is rather, *ahem* pathological, but in the end I don't think you made a circular construct. You just made a really deep set of nested hashes. I think the whole thing can unwind from the top. The reference you destroy is the lynchpin and once it is pulled each remaining level can fall one by one.

# These are bad, um-kay?
my $a;
$a = \$a;

my ($b,$c);
($b, $c) = \($c, $b);

my @d;
$d[0]=0;
$d[$_]=\$d[$_-1] for (1..100_000);
$d[0]=\$d[$#d];
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