open (CORPUS, "little_corpus.txt") or die "Warning! Can't open corpus file 
$!\n";

while ($line = <CORPUS>) {    
    chomp($line);   

    for ($i = 0; $i < length($line) - 1; $i++) {
        # The following stores the current character in a "hash table"
        $sequences{substr($line, $i, 2)} = 1 ;
    }
}
# Get a list of all the items in the inventory hash table
@found_seqs = keys %sequences;
print "List of all two-char sequences in the file (" . scalar(@found_seqs) . " total)\n";
foreach $seq (@found_seqs) {
    print "$seq "
}
print "\n";

open (TEST, "test_words.txt") or die "Warning! Can't open file of test words: $!\n";
while ($line = <TEST>) {
    chomp($line);
    $legal = 1;        
    for ($i = 0; $i < length($line) - 1; $i++) {
        if (not exists $sequences{substr($line, $i, 2)}) {
            print "Word $line is illegal (Sequence " . substr($line, $i, 2) ." not in corpus inventory)\n";
            $legal = 0;
            last;        
        }
    }
    if ($legal) {
        print "Word $line is legal\n";            
    }
}