Search for occurrences of contents of a file in another file - perl

Question

I want to search the contents of files in a directory for words present in files in another directory. Is there a better way to do it than the following? (By better mean memory usage wise)
More specifically:
folder 1 has several files, each file has several lines of text.
folder 2 has several files, each file has several words, each on its line.
What I want to do is count the number of occurrences of each word in each file in folder 2 in each line of each file of folder 1.
I hope that wasn't too confusing.
open my $output, ">>D:/output.txt";
my #files = <"folder1/*">;
my #categories = <"folder2/*">;
foreach my $file (#files){
open my $fileh, $file || die "Can't open file $companyName";
foreach my $line (<$fileh>){
foreach my $categoryName (#categories){
open my $categoryFile, $categoryName || die "Can't open file $categoryName";
foreach my $word(<$categoryFile>){
#search using regex
}
#print to output
}
}
}

Answer 1

One obvious improvement is to open all the category files first in a separate loop and cache the words in them into a hash of arrays (hash key being the filename), or just one big array if you don't care which search word came from which file.
This will avoid having to re-read the search files for every line in every $file - AND help get rid of duplicate search words in the bargain.
use File::Slurp;
open my $output, ">>D:/output.txt";
my %categories = ();
my #files = <"folder1/*">;
my #categories = <"folder2/*">;
foreach my $categoryName (#categories) {
my #lines = read_file($categoryName);
foreach my $category (#lines) {
chomp($category);
$categories{$category} = 0;
}
}
# add in some code to uniquify #categories
foreach my $file (#files) {
open my $fileh, $file || die "Can't open file $companyName";
foreach my $line (<$fileh>) {
foreach my $category (#categories) {
# count
}
}
# output
}
Also, if these are real "words" - meaning a category of "cat" needs to match "cat dog" but not "mcat" - I would count the word usage by splitting instead of a regex:
foreach my $line (<$fileh>) {
my #words = split(/\s+/, $line);
foreach my $word (#words) {
$categories{$word}++ if exists $categories{$word};
}
}