Here ya go:

my %dictionary = ();
open(DICTIONARY, "<your_dictionary") or die $!;
while (<DICTIONARY>) {
    chomp;
    $dict{$_} = 1;
}
close(DICTIONARY);

while (doc_to_check) {
    unless ($dict{$_}) {
        print "$_ is spelled incorrectly.\n";
    }
}
[download]

The key is finding a good dictionary, and providing alternative spellings when a word isn't found.