# from your add_words() sub
...
   $words{"$category-$word"} += $words_in_file{$word};

##</code><code>##

sub show_classification{
    my ( $wantedCategory ) = @_;

   #for each word in the matching category, print the word
    for my $key ( keys %words ) {
        my( $category, $word ) = split '-', $key;
        print "$word\n" if $category eq $wantedCategory;
    }
}