If you know what the third level's key should be, you can check for existence:

if (exists $hash->{A}->{B2}->{C}) { ...
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If you do not know, you will need to iterate:

for my $k (keys %{ $hash->{A}->{B2} } ){ ...
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Why don't you TRY the code you think should work (or fail), and if it behaves differently from your expectations, we can help explain why.

As for the thing you were attempting in your previous node:

   $hash->{'A'}->{'B2'}->{*}

#is actually Equivalent to
  my @subkeys = keys %{ $hash->{A}->{B2} };
#at least  - I think this is where you were heading...
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