On the other hand, if we assume that the square brackets in key3 are a typographic error, as it probably is, then the answer to the question would be straightforward, such as:

print $$hash{'key1'}{'key2'}{'key3'};    ==> will print "Hello"
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If you want to iterate through the structure, you might use the keys function, and, since this function requires a hash argument, you might have to tell Perl explicitly that this is what it is, e.g. noting the underlined text below:
keys %{$$hash{'key1'}{'key2'}}

Finally, note that $$hash{'key'} is a shorthand, a favorite of mine, that also could have been written $hash->{'key'}.

Notice that in my example the hash-keys are string literals as they must be.   If you had omitted the quotes, Perl would interpret them as so-called barewords, and, if you did not use use strict; use warnings;, you might not be advised of the issue.   Perl might interpret them as (undefined...) variables and produce nothing useful.

Data::Dumper is always your friend.

HTH ...