When setting something like this up there are 3 things to consider:
1. What is the steady state, normal thing causes the desired action?
2. How do we get it started? (what is special about it?)
3. How do we get it stopped? (what is special about it?)

One approach for is to dump the results and reset %HOH when the $key changes. To do that perhaps:

my $prev_key = "";
my %HOH;
while ( a line read from file )
{
   ...get the current key .. code that you have already
   if (  $current_key ne $prev_key)
   {
      if ($prev_key ne "" ) #don't dump first time through!
      {
         ...dump the %HOH  #this is the previous result
      }
      $prev_key = $current_key;
      ...%HOH=();  #reset!
   }
   
   ...processing of the current_key
}
...dump the %HOH ... #the last record...
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To solve question (3), after the loop ends due to lack of data, dump the final incarnation of the hash.

There is of course more than one way to accomplish what you need, but the 3 questions are always the same : (1)steady-state operation, (2)getting started, (3)getting finished.

I recommend writing the "steady-state", question (1) code first and then "fiddle with it" until questions (2) and (3) work out ok.

One quick question, how do I avoid having to save the huge dataset in memory by processing and printing the results on fly? I would like to undef the HASH at after each second in time i.e create a loop for each key and undef my %HOH.

For that, you want to introduce a simple control break into your program.