I'm posting this at the top, because otherwise it would be lost down in the level 5 of comments to comments to comments. The code (we learned later) looks like this:

while ( my @col_col = $sthZK_Update_extract->fetchrow() ) {
#stuff    
      my $SUM_xi = $col_col[5];
      my $SUM_ni = $col_col[6];
# more stuff
      print "\n^^^^^^\n$Key_m, $Aggregated_area, $Cause, $SUM_wixi, $S
+UM_wi, $SU
+M_xi, $SUM_ni, $Sum_wi2xi\n";
    }
    
    my $numerator_L = 2*($SUM_ni-$SUM_xi);
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Remember how I asked you to print the values JUST at the point were you were using them? Because I thought the values were not what you thought they were.

You are declaring variables with my - giving them scope inside the loop.

Once you are outside of the loop, the values are GONE.

If you had put the print statement just before the assign statement, you would have caught it yourself. If you had done use strict; Perl would have caught it for you. That is (one of) the reasons why use strict; is a really, really good idea.