Assuming you know the first key, you have a few simple options. The first first is to pull the keys from the nested hash and do a search through them.

my $max_event;
map {$max_event = $_ if $_ > $max_event} keys %{$hash{1}};
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You can then use $max_event to determine what your next event should be.

If you have to do this many times, I would suggest an additional field in your hash that tracks the max event like so:

%hash = (
          1 => {
                  1 => "", #dynamically created
                  name => "",
                  that => "",
                  current_max => 1, 
          },
          2.... # iteration
);
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Now if you need to know the current highest event, you just examine $hash{1}{current_max}. You'll have to ensure that you increment this field each time you add an event.

This approach could be a problem if the event could have a valid data field named current_max. In this case, you will have to modify your data structure to ensure that the current_max field is cleanly separated from the event data. But it would otherwise work the same.