So, is there a reason (technically, best practice, etc) that I should not just use this format:

How big is your monitor? :)

FWIW, a best practice policy might outlaw $$fa{$fa}{$fb}{$fc}{$fa}{$fq}{$fp} or even $fa->{$fa}{$fb}{$fc}{$fa}{$fq}{$fp} and demand intermediates or even helpers

{
    use Data::Diver qw/ Dive /;
    my $metric = Dive( $mainHash , $hashType , $regionID, 'MetricType'
+ );
    my $cirtem = Dive( $metric, $$resultHash{Location} , $yearMonthDat
+eList[$start] );
    $cirtem +=  $$resultHash{Value};
}
[download]

If you make $mainHash an object it might collapse into

{
   $mash->mertem( $mash->mertem + $value );
}
[download]