Close, yes, for some value of close ;-) Move the comparison into a loop.

Conditionals as keys to a hash don't work. Hash keys are strings. Ok, you have your conditions inside double quotes, and $grade is defined (but you should chomp it, since it contains a trailing newline), so you end up with a string that could be evaluated with eval. But what if "wibble" was entered? You would end up having

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

which isn't a valid comparison. You need to check the content of $grade first.

It seems from your code that you want to have the comparisons evaluated in the order they are listed. You can't operate directly on a hash for that without bringing the keys into order, since a hash's elements aren't necessarily (in fact, unlikely) returned in the order they were entered.

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

Here you are masking the $grade variable with a temporary loop variable. No way to get at what you've read from STDIN. And you are iterating over a flat list of key/value pairs.

If you want to iterate over the keys of your hash, use

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

To iterate over the values, use

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

One approach for your problem is reversing your hash and using anonymous subroutines as values:

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 $result,$/;
[download]

Note that 'F' is never returned, since for all  $grade <= 60 'D' will be returned. If you evaluate 'F' up front, none of A - D will ever be returned.

So maybe you have a typo, and you really mean your comparisons to be identical for all keys? That would simplify things, no need for anonymous subs, no need to reverse the hash:

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, $/;
[download]

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}