The following version uses the same logic as the original, is easy to understand (and modify), and works on very old versions of perl.

use strict;
use warnings;
my ($score) = @ARGV;
die "Score must be less than or equal to 100.\n" if $score > 100;

my $letter_grade = $score <=  39 ? 'F'
                 : $score <=  49 ? 'E'
                 : $score <=  59 ? 'D'
                 : $score <=  64 ? 'C'
                 : $score <=  69 ? 'C+'
                 : $score <=  74 ? 'B'
                 : $score <=  79 ? 'B+'
                 :                 'A'
                 ;

print "The student has gotten a $letter_grade for the score of $score\
+n";
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