For greatest efficiency, and minimal resource usage, make mysql do the heavy lifting. Get the domain counts by: 
my @domain_counts = GUI::DB::query ( $dbh,
    "SELECT date(now()), substr(addr,locate('@',addr)+1) as maildomain
+, count (*) as mailcount
     FROM mailing
     GROUP BY maildomain 
     ORDER BY mailcount DESC"
  );

[download]
After that, the storage, and top-50 selection becomes easy. Here is the relevant SQL... 
  INSERT INTO dailydomaincounts (maldate,maildomain,mailcount) VALUES 
+(?,?,?);

  SELECT SUM(mailcount) as TOTAL from  dailymailcounts 
          WHERE  maildate >= date(now()) - INTERVAL(30 days);

  SELECT maildomain, sum(mailcount) * 100.0 / $total as monthlymailpct
+ from dailymailcounts 
         WHERE maildate >= date(now()) - INTERVAL(30 days)
         GROUP BY maildomain
         ORDER BY monthlymailpct DESC
         LIMIT 50;

[download]
Of course, the e-mail splitting at the first '@' is not the worlds most robust implementation, but given the other artificial constraints imposed, it should suffice.
SQL not tested.

SQL "query complexity" is a rather vague, subjective term - I do not consider the above queries to be "complex".

             "I'm fairly sure if they took porn off the Internet, there'd only be one website left, and it'd be called 'Bring Back the Porn!'"
        -- Dr. Cox, Scrubs

In reply to Re: Please provide a hint for me to continue with the rest of my program by NetWallah
in thread Please provide a hint for me to continue with the rest of my program by pooyan

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