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