%hash = ();

##</code><code>##

my $hashref = {};
$$hashref{level1_key}{level2_key}{level3_key} = "value";
...

for my $key1 ( keys %$hashref ) {
    for my $key2 ( keys %{$$hasref{$key1}} ) {
        for my $key3 ( keys %{$$hasref{$key1}{$key2}} ) {
            # do something with $$hashref{$key1}{$key2}{$key3}
        }
    }
}