#!/usr/bin/env perl
 
use 5.014;
use warnings;
 
my %kw;   # $kw{kw}->{name} = Number of times kw appears in name.txt
my %name; # Inverse of %kw; $name{name}->{keyword}
 
read_words() for <*.txt>;
 
# Now we can do all sorts of useful things with the two hashes: 
say "$_ has " . (keys $name{$_}) . " unique words" for sort keys %name;
say '';
 
# Keywords ordered by occurrence count
for my $kw (sort { keys $kw{$b} <=> keys $kw{$a} } keys %kw) {
    my $count = keys $kw{$kw};
    printf "%10s appears in %2d file%s: %s\n",
        $kw, $count, $count > 1 ? 's' : ' ',
        join(', ', sort keys %{$kw{$kw}});
}

# Pull in the word lists.
sub read_words { 
    open my $fh, '<', $_ or die "Can't open $_: $!";
    my $name = s/\.txt$//r;
    
    while (<$fh>) {
        chomp;
        $kw{$_}->{$name}++;
        $name{$name}->{$_}++;
    }
    close $fh;
}