first reformat your statement:

my $up = "
    UPDATE  Prior_to_CI_calc_A_".$region." 
    SET     SUM_wi              = ".$SUM_Weight_wi."
    ,       Sum_wixi_x_1000     = ".$SUM_wixi_x_1000_fill."
    ,       Sum_wi2xi_x_1000    = ".$SUM_wi2xi_x_1000_fill."

    FROM    Prior_to_CI_calc_A_".$region." A

    WHERE   A.Aggregated_area   LIKE '".$Aggregated_area_fill."' 
    AND     A.Cause             LIKE '".$Cause_fill."'
";
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then rdfields is completey right:
>   There is no from clause in an update statement (except in sub-queries).
So what do you actually want to do?
pelagic