It's easier if you store the keys in an array. For instance:

my %hash;
my @path = (5, 6);

my $ref = $hash{root}{1} ||= {};
$ref = $$ref{$_} ||= {} for @path;
$$ref{2} = "b";

use YAML;
print Dump \%hash;
__END__
---
root:
  1:
    5:
      6:
        2: b
[download]

---

---

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