A subroutine with a series of returns is one way to make the logic simple and clear:

#!/usr/bin/env perl
use Modern::Perl;

sub figure_grade {
    my $n = shift;
    return undef if $n =~ /\D/ or $n > 100 or $n < 0;
    return 'A'   if $n > 79; # 80% == A?  Serious grade inflation!
    return 'B+'  if $n > 74;
    return 'B'   if $n > 69;
    return 'C+'  if $n > 64;
    return 'C'   if $n > 59;
    return 'D'   if $n > 49;
    return 'E'   if $n > 39;
    return 'F';
}

my $g = figure_grade($ARGV[0]);
if($g){
    say "The student has gotten the grade $g for the score of $ARGV[0]
+";
} else {
    say "Please enter a value between 0 and 100";
}
[download]