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

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

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

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

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

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