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foreach my $pair_ref ( @{ $r{$k} } ) {      # for each [col4,col6] pai
+r that matched on the four-column key
    my ($col4, $col6) = @$pair_ref;         # get the (col4,col6) valu
+es
    $x += $col4;                            # x is the summation of al
+l the matching col4 values
    $y += $col6;                            # similar for y
}