Perl list processing is for hashes, too

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This month I started a new job at Alert Logic, a cybersecurity provider with Perl (among many other things) at its beating heart. I’ve been learning a lot, and part of the process has been understanding the APIs in the code base. To that end, I’ve been writing small test scripts to tease apart data structures, using Perl array-processing, list-processing, and hash- (i.e., associative array)-processing functions.

I’ve covered map, grep, and friends a couple times before. Most recently, I described using List::Util’s any function to check if a condition is true for any item in a list. In the simplest case, you can use it to check to see if a given value is in the list at all:

use feature 'say';
use List::Util 'any';
my @colors =
  qw(red orange yellow green blue indigo violet);
say 'matched' if any { /^red$/ } @colors;

However, if you’re going to be doing this a lot with arbitrary strings, Perl FAQ section 4 advises turning the array into the keys of a hash and then checking for membership there. For example, here’s a simple script to check if the colors input (either from the keyboard or from files passed as arguments) are in the rainbow:

#!/usr/bin/env perl
use v5.22; # introduced <<>> for safe opening of arguments
use warnings;
 
my %in_colors = map {$_ => 1}
  qw(red…