Interesting. A little tweaking and we come up with the following which preserves the ordering and priority encoding nature of the original function. Note in particular that the addition is replaced by a bitwise or - that's where the priority encoding bit happens (it is also unaffected by duplicate flags). The bitwise complements preserve the original sort order.

my %flagvalues = (
  R => ~(2**1 - 1),
  T => ~(2**2 - 1),
  B => ~(2**3 - 1),
  I => ~(2**4 - 1),
  F => ~(2**5 - 1),
  L => ~(2**6 - 1),
);

sub flagValue {
  my $weight = ~(2**7 - 1);
 
  for (split //, shift){
    $weight |= $flagvalues{$_} if exists $flagvalues{$_};
  }
 
  return ~$weight;
}
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