plate of eggs and hash browns

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 orange yellow green blue indigo violet);

while (<<>>) {
  say "$_ is in the rainbow" if $in_colors{$_};

List::Util has a bunch of functions for processing lists of pairs that I’ve found useful when pawing through hashes. pairgrep, for example, acts just like grep but instead assigns $a and $b to each key and value passed in and returns the resulting pairs that match. I’ve used it as a quick way to search for hash entries matching certain value conditions:

use List::Util 'pairgrep';
my %numbers = (zero => 0, one => 1, two => 2, three => 3);
my %odds = pairgrep {$b % 2} %numbers;

Sure, you could do this by invoking a mix of plain grep, keys, and a hash slice, but it’s noisier and more repetitive:

use v5.20; # for key/value hash slice 
my %odds = %numbers{grep {$numbers{$_} % 2} keys %numbers};

pairgreps compiled C‑based XS code can also be faster, as evidenced by this Benchmark script that works through a hash made of the Unix words file (479,828 entries on my machine):

#!/usr/bin/env perl

use v5.20;
use warnings;
use List::Util 'pairgrep';
use Benchmark 'cmpthese';

my (%words, $count);
open my $fh, '<', '/usr/share/dict/words'
  or die "can't open words: $!";
while (<$fh>) {
  $words{$_} = $count++;
close $fh;

cmpthese(100, {
  grep => sub {
    my %odds = %words{grep {$words{$_} % 2} keys %words};
  pairgrep => sub {
    my %odds = pairgrep {$b % 2} %words;
} );

Benchmark output:

           Rate     grep pairgrep
grep     1.47/s       --     -20%
pairgrep 1.84/s      25%       --

In general, I urge you to work through the Perl documentations tutorials on references, lists of lists, the data structures cookbook, and the FAQs on array and hash manipulation. Then dip into the various list-​processing modules (especially the included List::Util and CPAN’s List::SomeUtils) for ready-​made functions for common operations. You’ll find a wealth of techniques for creating, managing, and processing the data structures that your programs need.

Friday, December 17, 2021, marked the thirty-​fourth birthday of the Perl programming language, and coincidentally this year saw the release of version 5.34. There are plenty of Perl developers out there who haven’t kept up with recent (and not-​so-​recent) improvements to the language and its ecosystem, so I thought I might list a batch. (You may have seen some of these before in May’s post Perl can do that now!”)

The feature pragma

Perl v5.10 was released in December 2007, and with it came feature, a way of enabling new syntax without breaking backward compatibility. You can enable individual features by name (e.g., use feature qw(say fc); for the say and fc keywords), or by using a feature bundle based on the Perl version that introduced them. For example, the following:

use feature ':5.34';

…gives you the equivalent of:

use feature qw(bareword_filehandles bitwise current_sub evalbytes fc indirect multidimensional postderef_qq say state switch unicode_eval unicode_strings);

Boy, that’s a mouthful. Feature bundles are good. The corresponding bundle also gets implicitly loaded if you specify a minimum required Perl version, e.g., with use v5.32;. If you use v5.12; or higher, strict mode is enabled for free. So just say:

use v5.34;

And lastly, one-​liners can use the -E switch instead of -e to enable all features for that version of Perl, so you can say the following on the command line:

perl -E 'say "Hello world!"'

Instead of:

perl -e 'print "Hello world!\n"'

Which is great when you’re trying to save some typing.

The experimental pragma

Sometimes new Perl features need to be driven a couple of releases around the block before their behavior settles. Those experiments are documented in the perlexperiment page, and usually, you need both a use feature (see above) and no warnings statement to safely enable them. Or you can simply pass a list to use experimental of the features you want, e.g.:

use experimental qw(isa postderef signatures);

Ever-​expanding warnings categories

March 2000 saw the release of Perl 5.6, and with it, the expansion of the -w command-​line switch to a system of fine-​grained controls for warning against dubious constructs” that can be turned on and off depending on the lexical scope. What started as 26 main and 20 subcategories has expanded into 31 main and 43 subcategories, including warnings for the aforementioned experimental features.

As the relevant Perl::Critic policy says, Using warnings, and paying attention to what they say, is probably the single most effective way to improve the quality of your code.” If you must violate warnings (perhaps because you’re rehabilitating some legacy code), you can isolate such violations to a small scope and individual categories. Check out the strictures module on CPAN if you’d like to go further and make a safe subset of these categories fatal during development.

Document other recently-​introduced syntax with Syntax::Construct

Not every new bit of Perl syntax is enabled with a feature guard. For the rest, there’s E. Choroba’s Syntax::Construct module on CPAN. Rather than having to remember which version of Perl introduced what, Syntax::Construct lets you declare only what you use and provides a helpful error message if someone tries to run your code on an older unsupported version. Between it and the feature pragma, you can prevent many head-​scratching moments and give your users a chance to either upgrade or workaround.

Make built-​in functions throw exceptions with autodie

Many of Perl’s built-​in functions only return false on failure, requiring the developer to check every time whether a file can be opened or a system command executed. The lexical autodie pragma replaces them with versions that raise an exception with an object that can be interrogated for further details. No matter how many functions or methods deep a problem occurs, you can choose to catch it and respond appropriately. This leads us to…

try/​catch exception handling and Feature::Compat::Try

This year’s Perl v5.34 release introduced experimental try/​catch syntax for exception handling that should look more familiar to users of other languages while handling the issues surrounding using block eval and testing of the special $@ variable. If you need to remain compatible with older versions of Perl (back to v5.14), just use the Feature::Compat::Try module from CPAN to automatically select either v5.34’s native try/​catch or a subset of the functionality provided by Syntax::Keyword::Try.

Pluggable keywords

The abovementioned Syntax::Keyword::Try was made possible by the introduction of a pluggable keyword mechanism in 2010’s Perl v5.12. So was the Future::AsyncAwait asynchronous programming library and the Object::Pad testbed for new object-​oriented Perl syntax. If you’re handy with C and Perl’s XS glue language, check out Paul LeoNerd” Evans’ XS::Parse::Keyword module to get a leg up on developing your own syntax module.

Define packages with versions and blocks

Perl v5.12 also helped reduce clutter by enabling a package namespace declaration to also include a version number, instead of requiring a separate our $VERSION = ...; v5.14 further refined packages to be specified in code blocks, so a namespace declaration can be the same as a lexical scope. Putting the two together gives you:

package Local::NewHotness v1.2.3 {

Instead of:

    package Local::OldAndBusted;
    use version 0.77; our $VERSION = version->declare("v1.2.3");

I know which I’d rather do. (Though you may want to also use Syntax::Construct qw(package-version package-block); to help along with older installations as described above.)

The // defined-​or operator

This is an easy win from Perl v5.10:

defined $foo ? $foo : $bar  # replace this
$foo // $bar                # with this


$foo = $bar unless defined $foo  # replace this
$foo //= $bar                    # with this

Perfect for assigning defaults to variables.

state variables only initialize once

Speaking of variables, ever want one to keep its old value the next time a scope is entered, like in a sub? Declare it with state instead of my. Before Perl v5.10, you needed to use a closure instead.

Save some typing with say

Perl v5.10’s bumper crop of enhancements also included the say function, which handles the common use case of printing a string or list of strings with a newline. It’s less noise in your code and saves you four characters. What’s not to love?

Note unimplemented code with ...

The ... ellipsis statement (colloquially yada-​yada”) gives you an easy placeholder for yet-​to-​be-​implemented code. It parses OK but will throw an exception if executed. Hopefully, your test coverage (or at least static analysis) will catch it before your users do.

Loop and enumerate arrays with each, keys, and values

The each, keys, and values functions have always been able to operate on hashes. Perl v5.12 and above make them work on arrays, too. The latter two are mainly for consistency, but you can use each to iterate over an array’s indices and values at the same time:

while (my ($index, $value) = each @array) {

This can be problematic in non-​trivial loops, but I’ve found it helpful in quick scripts and one-liners.

delete local hash (and array) entries

Ever needed to delete an entry from a hash (e.g, an environment variable from %ENV or a signal handler from %SIG) just inside a block? Perl v5.12 lets you do that with delete local.

Paired hash slices

Jumping forward to 2014’s Perl v5.20, the new %foo{'bar', 'baz'} syntax enables you to slice a subset of a hash with its keys and values intact. Very helpful for cherry-​picking or aggregating many hashes into one. For example:

my %args = (
    verbose => 1,
    name    => 'Mark',
    extra   => 'pizza',
# don't frob the pizza
$my_object->frob( %args{ qw(verbose name) };

Paired array slices

Not to be left out, you can also slice arrays in the same way, in this case returning indices and values:

my @letters = 'a' .. 'z';
my @subset_kv = %letters[16, 5, 18, 12];
# @subset_kv is now (16, 'p', 5, 'e', 18, 'r', 12, 'l')

More readable dereferencing

Perl v5.20 introduced and v5.24 de-​experimentalized a more readable postfix dereferencing syntax for navigating nested data structures. Instead of using {braces} or smooshing sigils to the left of identifiers, you can use a postfixed sigil-and-star:

push @$array_ref,    1, 2, 3;  # noisy
push @{$array_ref},  1, 2, 3;  # a little easier
push $array_ref->@*, 1, 2, 3;  # read from left to right

So much of web development is slinging around and picking apart complicated data structures via JSON, so I welcome anything like this to reduce the cognitive load.

when as a statement modifier

Starting in Perl v5.12, you can use the experimental switch features when keyword as a postfix modifier. For example:

for ($foo) {
    $a =  1 when /^abc/;
    $a = 42 when /^dna/;

But I don’t recommend when, given, or givens smartmatch operations as they were retconned as experiments in 2013’s Perl v5.18 and have remained so due to their tricky behavior. I wrote about some alternatives using stable syntax back in February.

Simple class inheritance with use parent

Sometimes in older object-​oriented Perl code, you’ll see use base as a pragma to establish inheritance from another class. Older still is the direct manipulation of the package’s special @ISA array. In most cases, both should be avoided in favor of use parent, which was added to core in Perl v5.10.1.

Mind you, if you’re following the Perl object-​oriented tutorial’s advice and have selected an OO system from CPAN, use its subclassing mechanism if it has one. Moose, Moo, and Class::Accessor’s antlers” mode all provide an extends function; Object::Pad provides an :isa attribute on its class keyword.

Test for class membership with the isa operator

As an alternative to the isa() method provided to all Perl objects, Perl v5.32 introduced the experimental isa infix operator:

# or
$my_object isa Local::MyClass

The latter can take either a bareword class name or string expression, but more importantly, it’s safer as it also returns false if the left argument is undefined or isn’t a blessed object reference. The older isa() method will throw an exception in the former case and might return true if called as a class method when $my_object is actually a string of a class name that’s the same as or inherits from isa()s argument.

Lexical subroutines

Introduced in Perl v5.18 and de-​experimentalized in 2017’s Perl v5.26, you can now precede sub declarations with my, state, or our. One use of the first two is truly private functions and methods, as described in this 2018 Dave Jacoby blog and as part of Neil Bowers’ 2014 survey of private function techniques.

Subroutine signatures

I’ve written and presented extensively about signatures and alternatives over the past year, so I won’t repeat that here. I’ll just add that the Perl 5 Porters development mailing list has been making a concerted effort over the past month to hash out the remaining issues towards rendering this feature non-​experimental. The popular Mojolicious real-​time web framework also provides a shortcut for enabling signatures and uses them extensively in examples.

Indented here-​documents with <<~

Perl has had shell-​style here-​document” syntax for embedding multi-​line strings of quoted text for a long time. Starting with Perl v5.26, you can precede the delimiting string with a ~ character and Perl will both allow the ending delimiter to be indented as well as strip indentation from the embedded text. This allows for much more readable embedded code such as runs of HTML and SQL. For example:

if ($do_query) {
    my $rows_deleted = $dbh->do(<<~'END_SQL', undef, 42);
      DELETE FROM table
      WHERE status = ?
    say "$rows_deleted rows were deleted."; 

More readable chained comparisons

When I learned math in school, my teachers and textbooks would often describe multiple comparisons and inequalities as a single expression. Unfortunately, when it came time to learn programming every computer language I saw required them to be broken up with a series of and (or &&) operators. With Perl v5.32, this is no more:

if ( $x < $y && $y <= $z ) { ... }  # old way
if ( $x < $y <= $z )       { ... }  # new way

It’s more concise, less noisy, and more like what regular math looks like.

Self-​documenting named regular expression captures

Perl’s expressive regular expression matching and text-​processing prowess are legendary, although overuse and poor use of readability enhancements often turn people away from them (and Perl in general). We often use regexps for extracting data from a matched pattern. For example:

if ( /Time: (..):(..):(..)/ ) {  # parse out values
    say "$1 hours, $2 minutes, $3 seconds";

Named capture groups, introduced in Perl v5.10, make both the pattern more obvious and retrieval of its data less cryptic:

if ( /Time: (?<hours>..):(?<minutes>..):(?<seconds>..)/ ) {
    say "$+{hours} hours, $+{minutes} minutes, $+{seconds} seconds";

More readable regexp character classes

The /x regular expression modifier already enables better readability by telling the parser to ignore most whitespace, allowing you to break up complicated patterns into spaced-​out groups and multiple lines with code comments. With Perl v5.26 you can specify /xx to also ignore spaces and tabs inside [bracketed] character classes, turning this:


…into this:

/ [d-e g-i 3-7]/xx
/[ ! @ " # $ % ^ & * () = ? <> ' ]/xx

Set default regexp flags with the re pragma

Beginning with Perl v5.14, writing use re '/xms'; (or any combination of regular expression modifier flags) will turn on those flags until the end of that lexical scope, saving you the trouble of remembering them every time.

Non-​destructive substitution with s///r and tr///r

The s/// substitution and tr/// transliteration operators typically change their input directly, often in conjunction with the =~ binding operator:

s/foo/bar/;  # changes the first foo to bar in $_
$baz =~ s/foo/bar/;  # the same but in $baz

But what if you want to leave the original untouched, such as when processing an array of strings with a map? With Perl v5.14 and above, add the /r flag, which makes the substitution on a copy and returns the result:

my @changed = map { s/foo/bar/r } @original;

Unicode case-​folding with fc for better string comparisons

Unicode and character encoding in general are complicated beasts. Perl has handled Unicode since v5.6 and has kept pace with fixes and support for updated standards in the intervening decades. If you need to test if two strings are equal regardless of case, use the fc function introduced in Perl v5.16.

Safer processing of file arguments with <<>>

The <> null filehandle or diamond operator” is often used in while loops to process input per line coming either from standard input (e.g., piped from another program) or from a list of files on the command line. Unfortunately, it uses a form of Perl’s open function that interprets special characters such as pipes (|) that would allow it to insecurely run external commands. Using the <<>> double diamond” operator introduced in Perl v5.22 forces open to treat all command-​line arguments as file names only. For older Perls, the perlop documentation recommends the ARGV::readonly CPAN module.

Safer loading of Perl libraries and modules from @INC

Perl v5.26 removed the ability for all programs to load modules by default from the current directory, closing a security vulnerability originally identified and fixed as CVE-20161238 in previous versions’ included scripts. If your code relied on this unsafe behavior, the v5.26 release notes include steps on how to adapt.

HTTP::Tiny simple HTTP/1.1 client included

To bootstrap access to CPAN on the web in the possible absence of external tools like curl or wget, Perl v5.14 began including the HTTP::Tiny module. You can also use it in your programs if you need a simple web client with no dependencies.

Test2: The next generation of Perl testing frameworks

Forked and refactored from the venerable Test::Builder (the basis for the Test::More library that many are familiar with), Test2 was included in the core module library beginning with Perl v5.26. I’ve experimented recently with using the Test2::Suite CPAN library instead of Test::More and it looks pretty good. I’m also intrigued by Test2::Harness’ support for threading, forking, and preloading modules to reduce test run times.

Task::Kensho: Where to start for recommended Perl modules

This last item may not be included when you install Perl, but it’s where I turn for a collection of well-​regarded CPAN modules for accomplishing a wide variety of common tasks spanning from asynchronous programming to XML. Use it as a starting point or interactively select the mix of libraries appropriate to your project.

And there you have it: a selection of 34 features, enhancements, and improvements for the first 34 years of Perl. What’s your favorite? Did I miss anything? Let me know in the comments.

man cutting tress using chainsaw

The Java world had an… interesting weekend when security researchers revealed on December 9 a vulnerability in the popular Apache Log4j 2 software library for recording and debugging events. Systems as diverse as Amazon Web Services, Apple iCloud, and the Minecraft video game could be exploited to run arbitrary code on a server merely by sending a specially-​crafted string of text. Information technology professionals have been scrambling ever since the initial disclosure to patch, upgrade, reconfigure, or otherwise protect affected servers. It’s bad, and past unpatched vulnerabilities like this have been responsible for the exposure of millions of people’s sensitive data.

Many Perl applications use the similarly-​named and ‑designed Log::Log4perl library, and the good news is that as far as I can tell the latter doesn’t suffer from the type of vulnerability described above. This doesn’t mean poorly-​written or ‑configured Perl-​based systems are immune to all exploits, just this particular one. You should be safe to continue using Log4perl unless someone has deliberately configured it otherwise, and in fact, my work uses it extensively.

You might be surprised to read me suggesting a logging framework after writing multiple articles espousing the Perl step debugger as an alternative. Log4perl developer Mike Schilli’s 2002 introduction to the package for came down on the opposite side of the argument. It can seem like one of those programmer religious issues like tabs vs. spaces, vim vs. Emacs, or Linux vs. Windows. (For the record, the correct answers are spaces, BBEdit, and macOS. 😉)

But in this case, you can and should have the best of both worlds—logging at different levels to appropriate destinations while still dropping into the interactive debugger when you need to do something trickier like examine program state or tweak a data structure on the fly. I use both techniques and only emphasize the advocacy of step debugging because it’s understood less.

woman using a laptop with her daughter

Inspired by my parents coming to visit at the end of the week, I thought I’d write about how Perl classes can have parents” as well, from which they inherit methods. Although it might seem on the surface as though there’s more than one way to do it, these techniques all share the same underlying mechanism.

Where it all BEGINs: @ISA

Perl classes are just repurposed packages, i.e., a namespace for variables and subroutines. The two key differences are:

If you wanted to do everything by hand at the lowest level, you could make a subclass at compile time like this:

package Local::MyChildClass;
BEGIN { # don't do this:
    require Local::MyParentClass;
    push @ISA, 'Local::MyParentClass';

Don’t do that though, because we have…

base and parent

In 1997 Perl 5.004_04 introduced the base pragma (back when Perl used that kind of versioning scheme; in these days of semantic versioning we’d call it version 5.4.4). It does the above BEGIN block in a single line:

use base 'Local::MyParentClass'; # don't do this unless you're also using fields

You might see use base in older code especially if it’s also using the fields pragma. However, Perl developers discourage both as the former silences certain module loading errors while the latter is at odds with the object-​oriented programming principle of encapsulation.

So use parent instead, which Perl has included since version 5.10.1 in 2009:

use parent 'Local::MyParentClass';

A couple of years ago my Newfold Digital colleague David Oswald created a fork of parent called parent::versioned that supports specifying the lowest version for superclasses. You call it like this:

use parent::versioned ['Local::MyParentClass' => 1.23];

Within an OO system

There are dozens of object-​oriented programming systems on CPAN that provide syntactic sugar and extra features to Perl’s minimal but flexible basics. Two of the more popular ones, Moose and Moo, offer an extends keyword that you should use instead of use parent so that your subclasses may take advantage of their features:

package Local::MyChildClass;
use Moo;
extends 'Local::MyParentClass';

Moose can also specify a required superclass version:

package Local::MyChildClass;
use Moose;
extends 'Local::MyParentClass' => {-version => 1.23};

Also, use the MooseX::NonMoose module when extending non-​Moose classes, again so you get Moose features even though your methods are coming from somewhere else:

package Local::MyMooseClass;
use Moose;
use MooseX::NonMoose;
extends 'Local::MyPlainParentClass';

The experimental Object::Pad module specifies a single superclass while defining the class name with an optional version. Per the author’s suggested file layout, including a required minimum version, it would look like:

use Object::Pad 0.41;
package Local::MyChildClass;
class Local::MyChildClass isa Local::MyParentClass 1.23;

Object::Pad and Corinna, its inspiration, are works in progress so this syntax isn’t set in stone. The latter’s designer Curtis Ovid” Poe blogged earlier this week about considering a more self-​consistent syntax.

Multiple inheritance vs. roles

To quote the Perl documentation, multiple inheritance often indicates a design problem, but Perl always gives you enough rope to hang yourself with if you ask for it.” All the techniques described above except for Object::Pad support multiple inheritance by specifying a list of superclasses. For example:

package Local::MyChildClass;
use parent qw(Local::MyParentClass1 Local::MyParentClass2);

If you’re using roles instead of or on top of superclasses (I’ve seen both situations) and your OO system doesn’t support them on its own, you can use the Role::Tiny module, first by describing your role in one package and then consuming it in another:

package Local::DoesSomething;
use Role::Tiny;


package Local::MyConsumer;
use Role::Tiny::With;
with 'Local::DoesSomething';



Moo::Role uses Role::Tiny under the hood and Moo can compose roles from either. The syntax for both Moo and Moose is similar:

package Local::DoesSomething;
use Moo::Role; # or "use Moose::Role;"


package Local::MyConsumer;
use Moo; # or "use Moose;"
with 'Local::DoesSomething';



Object::Pad specifies roles with the role keyword, and both classes and roles use does to consume them:

use Object::Pad 0.56;
package Local::DoesSomething;
role Local::DoesSomething does Local::DoesSomethingElse;


use Object::Pad 0.56;
package Local::MyConsumer;
class Local::MyConsumer does Local::DoesSomething;



The previous caveat about possible changes to this syntax applies.

Like parent, (sort of) like child

Of course, the whole point of inheritance or role consumption is so your child or consumer class can reuse functions and methods. Each of the techniques above has its ways of overriding that code, from the Perl built-​in SUPER pseudo-​class to Moose’s override and super keywords, to Moose’s and Moo’s method modifiers. (You can use the latter outside of Moo since it’s provided by Class::Method::Modifiers.)

I’ve written about choosing between overriding and modifying methods before, and when it comes to Moose and Moo code I’m now on the side of using the around method modifier if a method needs to call an inherited or consumed method of the same name. Object::Pad doesn’t have method modifiers (yet), so classes that use it will have to satisfy themselves with SUPER in their methods with an :override attribute that will throw an error if a parent doesn’t also provide the same method.

The Parent Wrap

In the end, your choice of Perl OO system will determine how (or whether) you handle inheritance and may even be a deciding factor. Which would you choose? And more importantly, have I made my parents proud with this post?

animal antler big close up

At my work, we extensively use the Moose object system to take care of what would ordinarily be very tedious boilerplate object-​oriented Perl code. In one part of the codebase, we have a family of classes that, among other things, map Perl methods to the names of various calls in a third-​party API within our larger organization. Those private Perl methods are in turn called from public methods provided by roles consumed by these classes so that other areas aren’t concerned with said API’s details.

Without going into too many specifics, I had a bunch of classes all with sections that looked like this:

sub _create_method    { return 'api_add'     }
sub _retrieve_method  { return 'api_info'    }
sub _search_method    { return 'api_list'    }
sub _update_method    { return 'api_update'  }
sub _cancel_method    { return 'api_remove'  }
sub _suspend_method   { return 'api_disable' }
sub _unsuspend_method { return 'api_restore' }

... # etc.

The values returned by these very simple methods might differ from class to class depending on the API call needed, and different classes might have a different mix of these methods depending on what roles they consume.

These methods had built up over time as developers had expanded the classes’ functionality, and this week it was my turn. I decided to apply the DRY (don’t repeat yourself) principle and create them from a simple hash table like so:

my %METHOD_MAP = (
  _create_method    => 'api_add',
  _retrieve_method  => 'api_info',
  _search_method    => 'api_list',
  _update_method    => 'api_update',
  _cancel_method    => 'api_remove',
  _suspend_method   => 'api_disable',
  _unsuspend_method => 'api_restore',

At first, I thought to myself, These look like private read-​only attributes!” So I wrote:

use Moose;


has $_ => (
  is       => 'ro',
  init_arg => undef,
  default  => $METHOD_MAP{$_},
) for keys %METHOD_MAP;

Of course, I’d have to move the classes’ with statements after these definitions so the roles they consume could see” these runtime-​defined attributes. But some of the methods used to read these are class methods (e.g., called as ClassName->foo() rather than $object->foo()), and Moose attributes are only set after the construction of a class instance.

Then I thought, Hey, Moose has a MOP (meta-​object protocol)! I’ll use that to generate these methods at runtime!”

my $meta = __PACKAGE__->meta;

while (my ($method, $api_call) = each %METHOD_MAP) {
    $meta->add_method( $method => sub {$api_call} );

The add_method documentation strongly encourage[s]” you to pass a metamethod object rather than a code reference, though, so that would look like:

use Moose::Meta::Method;

my $meta = __PACKAGE__->meta;

while (my ($method, $api_call) = each %METHOD_MAP) {
    $meta->add_method( $method = Moose::Meta::Method->wrap(
      sub {$api_call}, __PACKAGE__, $meta,

This was getting ugly. There had to be a better way, and fortunately there was in the form of Dave Rolskys MooseX::ClassAttribute module. It simplifies the above to:

use MooseX::ClassAttribute;

class_has $_ => (
  is      => 'ro',
  default => $METHOD_MAP{$_},
) for keys %METHOD_MAP;

Note there’s no need for init_arg => undef to prevent setting the attribute in the constructor. Although they’re still Moose attributes, they act like class methods so long as the class consumes the roles that require them after the attribute definitions.

Lastly, if we were using Moo as a lightweight alternative to Moose, I could have instead selected Toby Inksters MooX::ClassAttribute. Although it has some caveats, it’s pretty much the only alternative to our initial class method definitions as Moo lacks a meta-​object protocol.

The lesson as always is to check CPAN (or the appropriate mix of your language’s software repository, forums like Stack Overflow, etc.) for anything that could conceivably have application outside of your particular circumstances. Twenty-​five years into my career and I’m still leaping into code without first considering that someone smarter than me has already done the work.