Here's a solution that lets you use your ranges. Note: I tried to use List::Util::first instead of grep, but there appears to be a bug with the 'first()' function.

use strict;

sub rank {
 local $_ = shift;
 tr/OBAFGKM/1-7/;
 $_;
}

# Enter questions and scores and answers
# Enter answers as a range of values.
my %scores = (
 1 => { 1   => { O9=>'B1',
                 K1=>'K2',
               },
        0.5 => { B4=>'B5',
                 B7=>'B7',
               },
      },
 2 => { 1   => { K8=>'K9',
               },
        0.5 => { F1=>'F2',
               },
      },
);

my %ranges;
for my $question (keys %scores) {
  my $scores = $scores{$question};
  my @ranges;
  for my $score (keys %$scores) {
   my $ranges = $scores->{$score};
   push @ranges, map { [ $score, rank($_), rank($ranges->{$_}) ]
                     } keys %$ranges;
  }
  @ranges = sort {$a->[1] cmp $b->[1]} @ranges;
  $ranges{$question} = \@ranges;
}

my %answers = (1 => 'K2', 2=>'F1');
my $total = 0;
while (my ($qu, $ans) = each %answers) {
 $ans = rank($ans);
 my ($aref) = grep { $_->[1] le $ans and $ans le $_->[2]
                   } @{$ranges{$qu}};
 $total += $aref->[0] if defined $aref;
}
print $total,"\n";
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