I was playing around with this and noticed a harmless but problematic side-effect of the way you construct the hash.

%scores = (
   map { $_ =>  1 } qw( A E I L N O R S T U ),
   map { $_ =>  2 } qw( B D G               ),
   map { $_ =>  3 } qw( C M P               ),
   map { $_ =>  4 } qw( F H V W Y           ),
   map { $_ =>  5 } qw( K                   ),
   map { $_ =>  8 } qw( J X                 ),
   map { $_ => 10 } qw( Q Z                 ),
);
print join($", sort keys %scores), $/;

__END__
10 2 3 4 5 8 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
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Since you're filtering for letters, the numbers don't get picked up, but the maps are having a side-effect mapping of the other maps below them. Parens around the maps seems to fix it.

I also learned something from your example. I would have predicted that the last unless defined would not work right on empty input b/c $_ still contains a "\n" which would be defined. I would have been wrong.