dd \%master_letter_freq;      # <-- QUERY
dd \@result;                  # <-- QUERY
RESULT: foreach (@result)
        {
            print "|$_|\n";   # <-- QUERY
            my %seen;
            $seen{$_}++ for (split //,lc $_);
            dd \%seen;        # <-- QUERY
            # ... and so on ...