my $t = tie(%hash, Tie::IxHash);
foreach my $line (@file){
    $line_count++;
    my @ngrams=produce_ngrams($line);
    foreach my $ngram (@ngrams) {
        #$t->Push(@{ $hash{$ngram} } => $line_count); 
        push(@{ $hash{$ngram} }, $line_count);
    }
}