hurray I found a solution, by adding : , (SELECT @prev_discussion:=0) pd, (SELECT @recent_post_rank:=0) rpr as follows: 
    SELECT x.group_discussion_id, x.comment, x.date_posted, 
        x.username, x.realname,
        TO_DAYS(NOW()) - TO_DAYS(x.date_posted) AS date_posted_days
    FROM (
          SELECT gdp.group_discussion_post_id, 
              gdp.group_discussion_id, gdp.comment, 
              gdp.date_posted, m.username, m.realname, 
          @recent_post_rank := IF
                  (
                      @prev_discussion = gdp.group_discussion_id, 
                      @recent_post_rank + 1, 
                      1
                  ) AS recent_post_rank, 
          @prev_discussion := gdp.group_discussion_id
      FROM group_discussion_posts gdp 
          JOIN members m 
          ON m.memberid = gdp.memberid, 
          (SELECT @prev_discussion:=0) pd, 
          (SELECT @recent_post_rank:=0) rpr 
          WHERE gdp.active = 1 
          AND gdp.reply_to_post_id != 0
          AND gdp.group_discussion_id IN (1, 2, 3)  
      ORDER BY gdp.group_discussion_id DESC, 
              gdp.group_discussion_post_id DESC
          ) x
      WHERE recent_post_rank <= 3

[download]
Hopefully this might help someone else in the future. I'm not sure exactly why it works, but it creates a subquery to select each user defined variable, and an alias for each (which is never used).

In reply to Re^2: Perl DBI not storing @prev_value in query by Beaker
in thread Perl DBI not storing @prev_value in query by Beaker

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