%scores = (
   "wibble<=90" => "A",
   ...
);

##</code><code>##

$grade = <STDIN>;
...
foreach $grade (%scores) {

##</code><code>##

foreach my $key (keys %hash) {
    ...
}

##</code><code>##

foreach my $value (values %hash) {
    ...
}

##</code><code>##

chomp (my $grade = <STDIN>);
$grade !~ /^\d+$/ and die "Input must be numeric!\n";
my %scores = (
    A => sub { return $_[0] <= 90 ? 1 : 0 },
    B => sub { return $_[0] <= 80 ? 1 : 0 },
    C => sub { return $_[0] <= 70 ? 1 : 0 },
    D => sub { return $_[0] <= 60 ? 1 : 0 }, # just a guess, otherwise like 'B'
    F => sub { return $_[0] >= 50 ? 1 : 0 }, # really? shouldn't that be '<=' ?
);
my $result;
foreach my $key (qw(D C B A F)) {
    if ($scores{$key}->($grade) ) {
        $result = $key;
        last;
    }
}
print $result,$/;

##</code><code>##

chomp (my $grade = <STDIN>);
$grade !~ /^\d+$/ and die "Input must be numeric!\n";

# hash with upper threshold as keys
my %scores = (
    90 => 'A',
    80 => 'B',
    70 => 'C',
    60 => 'D',
    50 => 'F',
);
my $result;
foreach my $key (sort keys %scores) {
    if ($grade <= $key) {
        $result = $scores{$key};
        last;
    }
}
print $result, $/;