This is a data problem. Data problems should be solved by the data layer and not the application layer. So, even if you could do this in Perl (which you can), you should be doing it in T-SQL (which, I hope, isn't much different from SQL.)

What you're looking for is a trigger. In Oracle, I would write it something like:

CREATE TRIGGER TRG_SUM_PERSONCOUNT
AFTER INSERT OR UPDATE OR DELETE ON CASES
BEGIN
    UPDATE geo_m_count g
       SET person_count = (
            SELECT COUNT(*)
              FROM cases c
             WHERE g.oacode LIKE c.oacode
               AND g.sex = c.sex
               AND c.ageyears BETWEEN g.age_start and g.age_end
           )
END;
/
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(The ending slash is important, for Oracle. YMMV)

Basically, what that does is whenever any of the rows in CASES changes, person_count is updated in all rows in GEO_M_COUNT. Now, this can be very inefficient, if you are doing more changes than selects. You might want to consider changing how you are viewing your data. Without knowing much, I am going to hazard a guess that you're doing some sort of statistical analysis of death information. Based on that, I would recommend looking at things differently. I'd have the following tables:

• CAUSE_OF_DEATH
  • ID
  • DESCRIPTION
• OA_CODE
  • ID
  • DESCRIPTION
• CASES
  • ID
  • GENDER
  • DATE_BIRTH
  • DATE_DEATH
  • OA_CODE - REFERENCES OA_CODE(ID)
  • CAUSE_OF_DEATH - REFERENCES CAUSE_OF_DEATH(ID)

In every base table, there is an ID field. This is a database unique identifier, assigned when the row is inserted. It has nothing to do with the data, other than to identify a given row.

Your other table (Geo_m_count) is a summary table. It doesn't hold data - it organizes it. So, you could do a view (materialized or not - your choice). Look up how views are done in MS-SQL. (I've never really used it, being an Oracle person.) But, your basic select statement could look like:

SELECT cause_of_death
      ,oa_code
      ,gender
      ,MIN( date_death - date_birth ) AS age_start
      ,MAX( date_death - date_birth ) AS age_end
      ,COUNT( * )
  FROM cases
 GROUP BY cause_of_death
         ,oa_code
         ,gender
;
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A few thoughts on your SQL in general.

1. You should really consider making your naming convention a little easier to use. For example, OAcode isn't very person-friendly. I'm sure there's a better name. Others:
   • Gender is better than Sex
   • Age is better than Ageyears (Age is almost always in years)
2. It's better to store DATEs instead of INTs, when dealing with things that span time, like Age_start and Age_end
3. This is just convention, but I would strongly recommend keeping all names in SQL in uppercase. Many databases (such as Oracle) are case-insensitive, but I wouldn't depend on it. Also, it makes your names stand out.
4. I hope you're specifying things like a PRIMARY KEY or a useful set of foreign key(s) for each table. Constraints are probably the most important feature of relational databases - they are the relational part. Without constraints, your data doesn't relate one table to the other. For example, you have OAcode in both tables. I would hope that OAcode is foreign-keyed to some other table that lists all the legal OAcode values. Otherwise, I 100% guarantee that you will eventually have bad data.