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my %rules = (A => 1,B => 1,C => 0,D => 1);    
my $s = 0;
while ($F[2] =~ /(\d+)([ABCD])/g) {    
...
    my $op = $2;
    $s += $n * $rules{$op};    
}

$s - Values from D at both ends are added to the total - if D does not
+ exist at both ends in any given code, whichever D is present is adde
+d regardless (the way the code currently works)

$s1 - Only the value from D at the start of the code is added

$s2 - Only the value from D at the end of the code is added

$s = 47 (2+40+1+4)
$s1 = 43 (2+40+1)
$s2 = 45 (40+1+4)

$s = 43 (2+40+1)
$s1 = 43 (2+40+1)
$s2 = 41 (40+1)