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 [email protected] 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

And:

$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:

$my_object->isa('Local::MyClass')
# 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 = $dbh->do(<<~'END_SQL', undef, 42);
      DELETE FROM table
      WHERE status = ?
      END_SQL
    say "$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:

/[d-eg-i3-7]/
/[[email protected]"#$%^&*()=?<>']/

…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-​2016 – 1238 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.

Look, I get it. You don’t like the Perl pro­gram­ming lan­guage or have oth­er­wise dis­re­gard­ed it as dead.” (Or per­haps you haven’t, in which case please check out my oth­er blog posts!) It has weird noisy syn­tax, mix­ing reg­u­lar expres­sions, sig­ils on vari­able names, var­i­ous braces and brack­ets for data struc­tures, and a menagerie of cryp­tic spe­cial vari­ables. It’s old: 34 years in December, with a his­to­ry of (some­times ama­teur) devel­op­ers that have used and abused that syn­tax to ship code of ques­tion­able qual­i­ty. Maybe you grudg­ing­ly accept its util­i­ty but think it should die grace­ful­ly, main­tained only to run lega­cy applications.

But you know what? Perl’s still going. It’s had a steady cadence of year­ly releas­es for the past decade, intro­duc­ing new fea­tures and fenc­ing in bad behav­ior while main­tain­ing an admirable lev­el of back­ward com­pat­i­bil­i­ty. Yes, there was a too-​long adven­ture devel­op­ing what start­ed as Perl 6, but that lan­guage now has its own iden­ti­ty as Raku and even has facil­i­ties for mix­ing Perl with its native code or vice versa.

And then there’s CPAN, the Comprehensive Perl Archive Network: a continually-​updated col­lec­tion of over 200,000 open-​source mod­ules writ­ten by over 14,000 authors, the best of which are well-​tested and ‑doc­u­ment­ed (apply­ing peer pres­sure to those that fall short), pre­sent­ed through a search engine and front-​end built by scores of con­trib­u­tors. Through CPAN you can find dis­tri­b­u­tions for things like:

All of this is avail­able through a mature instal­la­tion tool­chain that doesn’t break from month to month.

Finally and most impor­tant­ly, there’s the glob­al Perl com­mu­ni­ty. The COVID-​19 pan­dem­ic has put a damper on the hun­dreds of glob­al Perl Mongers groups’ mee­tups, but that hasn’t stopped the year­ly Perl and Raku Conference from meet­ing vir­tu­al­ly. (In the past there have also been year­ly European and Asian con­fer­ences, occa­sion­al for­ays into South America and Russia, as well as hackathons and work­shops world­wide.) There are IRC servers and chan­nels for chat, mail­ing lists galore, blogs (yes, apart from this one), and a quirky social net­work that pre­dates Facebook and Twitter.

So no, Perl isn’t dead or even dying, but if you don’t like it and favor some­thing new­er, that’s OK! Technologies can coex­ist on their own mer­its and advo­cates of one don’t have to beat down their con­tem­po­raries to be suc­cess­ful. Perl hap­pens to be battle-​tested (to bor­row a term from my friend Curtis Ovid” Poe), it runs large parts of the Web (speak­ing from direct and ongo­ing expe­ri­ence in the host­ing busi­ness here), and it’s still evolv­ing to meet the needs of its users.

woman in black tank top and blue denim jeans

This blog has devot­ed a fair amount of atten­tion to the pop­u­lar and mul­ti­fac­eted object-​oriented sys­tem Moose and its light­weight sub­set Moo. I’ve also cov­ered Object::Pad, the test­bed of con­cepts and syn­tax for Corinna, the pro­posed next-​generation Perl core OO sys­tem. But what if your project is too memory‑, performance‑, or dependency-​constrained for these options?

It turns out that CPAN has a rich his­to­ry of lighter-​weight OO mod­ules to meet many dif­fer­ent needs. If you can live with their trade-​offs, they’re worth inves­ti­gat­ing instead of rolling your own lay­er over Perl’s OO. Here are a few.

Class::Struct

Class::Structs main claim to fame is its inclu­sion in the stan­dard Perl dis­tri­b­u­tion, so there’s no need to install depen­den­cies from CPAN. It pro­vides a syn­tax for defin­ing class­es as C‑style structs at either com­pile time or run­time. (There’s no speed advan­tage to the for­mer; it just means that your class will be built as if you had writ­ten the acces­sors your­self as subs.) Here’s an example:

#!/usr/bin/env perl

use v5.24; # for strict, say, and postfix dereferencing
use warnings;

package Local::MyClass;
use Class::Struct (
    foo => '$',
    bar => '@',
    baz => '%',
);

package main;

my $obj = Local::MyClass->new(
    foo => 'hello',
    bar => [1, 2, 3],
    baz => { name => 'Mark'},
);

say $obj->foo, ' ', $obj->baz('name');
say join ',', $obj->bar->@*;

# replace the name element of baz
$obj->baz(name => 'Sharon');

# replace the second element of bar
$obj->bar(1, 'replaced');
say $obj->foo, ' ', $obj->baz('name');
say join ',', $obj->bar->@*;

And here’s the output:

hello Mark
1,2,3
hello Sharon
1,replaced,3

Note that Class::Struct sup­ports acces­sors for scalar, array, and hash types, as well as oth­er class­es (not demon­strat­ed). Consult the module’s doc­u­men­ta­tion for the dif­fer­ent ways to define and retrieve them.

Class::Accessor

Class::Accessor does one thing: it makes acces­sors and muta­tors (also known as get­ters and set­ters) for fields in your class. Okay, it actu­al­ly does anoth­er thing: it pro­vides your class with a new method to ini­tial­ize those fields. Those acces­sors can be read-​write, read-​only, or write-​only. (Why would you want write-​only acces­sors?) You can define any of them using either its his­tor­i­cal class meth­ods or a Moose-​like attribute syn­tax.

If you’re try­ing to squeeze every bit of per­for­mance out of your code and can sac­ri­fice a lit­tle flex­i­bil­i­ty in alter­ing acces­sor behav­ior, you can opt for Class::Accessor::Fast or Class::Accessor::Faster. The for­mer still uses hash ref­er­ences under the hood to rep­re­sent objects and the lat­ter uses array ref­er­ences. The main Class::Accessor doc­u­men­ta­tion con­tains an effi­cien­cy com­par­i­son of the three for your edification.

Here’s an exam­ple script using Class::Accessor::Faster and the Moose-​like syntax:

#!/usr/bin/env perl

use v5.12; # for strict and say
use warnings;

package Local::MyClass;
use Class::Accessor::Faster 'moose-like';

has readwrite => (is => 'rw');
has readonly  => (is => 'ro');

package main;

my $obj = Local::MyClass->new( { # must be a hash reference
    readwrite => 'hello',
    readonly  => 'world',
} );

say $obj->readwrite, ' ', $obj->readonly;
$obj->readwrite('greetings');
say $obj->readwrite, ' ', $obj->readonly;

# throws an error
$obj->readonly('Cleveland');

And here is its output:

hello world
greetings world
'main' cannot alter the value of 'readonly' on objects of class 'Local::MyClass' at ./caf.pl line 24.

Class::Tiny

Class::Tiny both does less and more than Class::Accessor. All of its gen­er­at­ed acces­sors are read-​write, but you can also give their attrib­ut­es lazy defaults. Its gen­er­at­ed con­struc­tor takes argu­ments via either a Class::Accessor-style hash ref­er­ence or a plain list of key/​value pairs, so that’s a lit­tle more con­ve­nient. It also sup­ports Moose-​style BUILDARGS, BUILD, and DEMOLISH meth­ods for argu­ment adjust­ment, val­i­da­tion, and object cleanup, respectively.

It’s a toss-​up as to which of the pre­vi­ous two is bet­ter.” You’ll have to exam­ine their respec­tive fea­tures and deter­mine which ones map to your needs.

Here’s an exam­ple script that shows a few of Class::Tiny’s unique features:

#!/usr/bin/env perl

use v5.12; # for strict and say
use warnings;

package Local::MyClass;
use Class::Tiny qw<foo bar>,
{
    baz       => 'default baz',
    timestamp => sub { time },
};

package main;

my $obj = Local::MyClass->new( # plain key-values OK
    foo => 'hello',
    bar => 'world',
);

say $obj->foo, ' ', $obj->bar;
say 'Object built on ', scalar localtime $obj->timestamp;
$obj->foo('greetings');
$obj->bar('Cleveland');
say $obj->foo, ' ', $obj->bar;
say $obj->baz;

And its output:

hello world
Object built on Tue Sep  7 09:00:00 2021
greetings Cleveland
default baz

Object::Tiny

For an even more min­i­mal­ist approach, con­sid­er Object::Tiny. Its acces­sors are read-​only, it gives you a sim­ple con­struc­tor, and that’s it. Its doc­u­men­ta­tion lists a num­ber of rea­sons why it can be supe­ri­or to Class::Accessor, includ­ing low­er mem­o­ry usage and less typ­ing. There’s also a fork called Object::Tiny::RW that adds read-​write sup­port to its accessors.

Class::Tiny’s doc­u­men­ta­tion con­tains a fea­ture table com­par­i­son of it, Object::Tiny, and Class::Accessor. This may help you decide which to use.

Here’s an exam­ple script:

#!/usr/bin/env perl

use v5.12; # for strict and say
use warnings;

package Local::MyClass;
use Object::Tiny qw<foo bar>;

package main;

my $obj = Local::MyClass->new(
    foo => 'hello',
    bar => 'world',
);

say $obj->foo, ' ', $obj->bar;

# has no effect unless you use Object::Tiny::RW
$obj->foo('greetings');
say $obj->foo, ' ', $obj->bar;

And its output:

hello world
hello world

Add some speed with XS

If the above options are still too slow and you don’t mind requir­ing a C com­pil­er to install them, there are vari­ants that use Perl’s XS inter­face instead of pure Perl code:

Roles with Role::Tiny

If you’re eye­ing Moose and Moo’s sup­port for roles (also known as traits) as an alter­na­tive to inher­i­tance but still want to keep things light with one of the above mod­ules, you’re in luck. The Role::Tiny mod­ule lets you com­pose meth­ods into con­sum­ing class­es with Moo-​like syn­tax and will pull in Common Lisp Object System-style method mod­i­fi­er sup­port from Class::Method::Modifiers if you need it. It does mean anoth­er cou­ple of CPAN depen­den­cies, so if that’s a prob­lem in your sit­u­a­tion you’ll just have to live with­out roles.

Here’s an exam­ple script with a role and a con­sum­ing class that uses Class::Tiny. The role requires that its con­sumers imple­ment a required_method, pro­vides a foo method that uses it, and a method mod­i­fi­er for bar.

#!/usr/bin/env perl

use v5.12; # for strict and say
use warnings;

package Local::MyRole;
use Role::Tiny;

requires 'required_method';

sub foo {
    my $self = shift;
    say $self->required_method();
}

before bar => sub {
    warn 'About to call bar...';
};

package Local::MyClass;
use Class::Tiny {name => ''};
use Role::Tiny::With;
with 'Local::MyRole';

sub bar {
    my ($self, $greeting) = @_;
    say "$greeting ", $self->name;
}

sub required_method {
    my $self = shift;
    return 'Required by Local::MyRole';
}

package main;

my $obj = Local::MyClass->new(name => 'Mark');
$obj->bar('hello');

$obj->name('Sharon');
$obj->bar('salutations');

$obj->foo();

And its output:

About to call bar... at ./rt.pl line 17.
hello Mark
About to call bar... at ./rt.pl line 17.
salutations Sharon
Required by Local::MyRole

What’s your favorite?

There will always be those who insist on writ­ing every­thing long­hand, but mod­ules like these can save a lot of time and typ­ing as well as reduce errors. Do you have a favorite, maybe some­thing I missed? Let me know in the comments.

Introduction: The current state of play

Perl has very min­i­mal” sup­port for object-​oriented (OO) pro­gram­ming out of the box by its own admis­sion. It’s class-​based but class­es are just pack­ages used dif­fer­ent­ly. Objects are just data struc­tures blessed into a class, meth­ods are just sub­rou­tines whose first argu­ment is an object or class name, and attributes/​properties are often just the key-​value pair of a hash stored in the object. (This last is a fea­ture shared with JavaScript, whose prototype-​based objects are just col­lec­tions of key-​value pairs with the keys addressed as prop­er­ties.) You’ve got poly­mor­phism, inher­i­tance, and it’s up to you to enforce encap­su­la­tion.

This can take a lot of work to use effec­tive­ly. To help address that, sev­er­al sys­tems have been devel­oped over the years to reduce boil­er­plate and pro­vide mod­ern (or post­mod­ern”) OO fea­tures that devel­op­ers from oth­er lan­guages expect. My favorite for a while has been Moo: it’s got the fea­tures I need 90% of the time like built-​in con­struc­tors, roles (an alter­na­tive to com­po­si­tion through inher­i­tance), attrib­ut­es, type val­i­da­tion, and method mod­i­fiers for enhanced poly­mor­phism. And if I need to dig around in the guts of class­es, attrib­ut­es, and the like I can always upgrade to Moo’s big broth­er Moose and its meta-​object pro­to­col with min­i­mal effort.

Corinna, Object::Pad, and porting dbcritic

But there’s a new kid on the block. Curtis Ovid” Poe has been spear­head­ing Corinna, an effort to bring effec­tive OO to the Perl core and leapfrog [empha­sis his] the capa­bil­i­ties of many OO lan­guages today.” No CPAN mod­ules, no chain of depen­den­cies; just sol­id OO fea­tures and syn­tax built-​in. And while Corinna is a ways off from ship­ping, Paul LeoNerd” Evans (maybe I should get a cool nick­name too?) has been imple­ment­ing some of these ideas as new Perl key­word syn­tax in his Object::Pad module.

Both Ovid and LeoNerd have been ask­ing devel­op­ers to try out Object::Pad, not just as a new toy, but to get feed­back on what works and what needs to be added. So I thought I’d try port­ing an old­er small Moo-​based project named dbcrit­ic to this new real­i­ty. In the process, I learned some of the advan­tages and dis­ad­van­tages of work­ing with Object::Pad. Hopefully, this can inform both it and Corinna’s evo­lu­tion as well as oth­er curi­ous devel­op­ers’ eval­u­a­tions. You can fol­low my cod­ing efforts in this GitHub branch.

First, the mar­quee result: the code for App::DBCritic (the class I start­ed with) is clean­er and short­er, with 33 lines shaved off so far. Mainly this is due to Object::Pad’s more con­cise attribute syn­tax (called slots” in its doc­u­men­ta­tion) and lack of explic­it sup­port for Moo’s attribute coer­cion. I only used the lat­ter for one attribute in the Moo ver­sion and I’m not sure it worked par­tic­u­lar­ly well, so it was­n’t hard to jet­ti­son. But if your code sup­ports coer­cions exten­sive­ly, you’ll have to look into Object::Pad’s BUILD or ADJUST phase blocks for now.

Before, a Moo attribute with var­i­ous options:

has schema => (
    is        => 'ro',
    coerce    => 1,
    lazy      => 1,
    default   => \&_build_schema,
    coerce    => \&_coerce_schema,
    predicate => 1,
);

After, an Object::Pad slot. No coer­cion and builder code is han­dled in a lat­er ADJUST block:

has $schema :reader :param = undef;

Speaking of ADJUST blocks, it took a lit­tle bit of insight from the #perl IRC chan­nel to real­ize that they were the appro­pri­ate place for set­ting slot defaults that are com­put­ed from oth­er slots. Previously I was using a maze of depen­den­cies mix­ing Moo lazy attrib­ut­es and builder meth­ods. Clarifying the main set of option­al con­struc­tor argu­ments into a sin­gle ADJUST block helped untan­gle things, so this might be an indi­ca­tion that lazy attrib­ut­es are an antipat­tern when try­ing to write clean code. It’s also worth not­ing that Object::Pad ADJUST blocks run on object con­struc­tion, where­as Moo lazy attrib­ut­es are only built when need­ed. This tends to mat­ter for data­base access.

The ADJUST block for the $schema slot:

ADJUST {
    my @connect_info = ( $dsn, $username, $password );
    if ($class_name and eval "require $class_name") {
        $schema = $class_name->connect(@connect_info);
    }
    elsif ( not ( blessed($schema) and $schema->isa('DBIx::Class::Schema') ) ) {
        local $SIG{__WARN__} = sub {
            if ( $_[0] !~ / has no primary key at /ms ) {
                print {*STDERR} $_[0];
            }
        };
        $schema = App::DBCritic::Loader->connect(@connect_info);
    }
    croak 'No schema defined' if not $schema;
}

Object::Pad’s slots have one great advan­tage over Moo and Moose attrib­ut­es: they direct­ly sup­port Perl array and hash data struc­tures, while the lat­ter only sup­ports scalars and ref­er­ences con­tained in scalars. This means meth­ods in your class can elim­i­nate a deref­er­enc­ing step, again lead­ing to clean­er code. I used this specif­i­cal­ly in the @violations array and %elements hash slots and was very pleased with the results.

The @violations and %elements slots and their ADJUST blocks:

has %elements;

ADJUST {
    %elements = (
        Schema       => [$schema],
        ResultSource => [ map { $schema->source($_) } $schema->sources ],
        ResultSet    => [ map { $schema->resultset($_) } $schema->sources ],
    );
}

has @violations;

ADJUST {
    @violations = map { $self->_policy_loop( $_, $elements{$_} ) }
        keys %elements;
}

method violations { wantarray ? @violations : \@violations }

Issues

I did have some devel­op­ment life­cy­cle issues with Object::Pad, but they’re main­ly a result of its future-​facing syn­tax. I had to give up using perltidy and perlcritic in my build and test phas­es, respec­tive­ly: perltidy does­n’t under­stand slot attrib­ut­es like :reader and :param and will emit an error file (but code still com­piles), and sev­er­al of the perlcritic poli­cies I use report prob­lems because its PPI pars­er does­n’t rec­og­nize the new syn­tax. I could add excep­tions in the perlcriticrc file and lit­ter my code with more ## no critic anno­ta­tions than it already had, but at this point, it was eas­i­er to just dis­able it entirely.

Another thing I had to dis­able for now was my Dist::Zilla::Plugin::Test::UnusedVars-gen­er­at­ed Test::Vars test for detect­ing unused vari­ables, as it reports mul­ti­ple fail­ures for the hid­den @(Object::Pad/slots) vari­able. It does have options for ignor­ing cer­tain vari­ables, though, so I can explore using those and pos­si­bly file a pull request to ignore that vari­able by default.

Conclusion: The future looks bright

Overall I’m sat­is­fied with Object::Pad and by exten­sion some of the syn­tax that Corinna will intro­duce. I’m going to try port­ing the rest of dbcrit­ic and see if I can work around the issues I list­ed above with­out giv­ing up the kwali­tee improve­ment tools I’m used to. I’ll post my find­ings if I feel it mer­its anoth­er blog.

What do you think? Is this the future of object-​oriented Perl? Let me know in the com­ments below.

I pub­lish Perl sto­ries on this blog once a week, and it seems every time there’s at least one response on social media that amounts to, I hate Perl because of its weird syn­tax.” Or, It looks like line noise.” (Perl seems to have out­last­ed that one — when’s the last time you used an acoustic modem?) Or the quote attrib­uted to Keith Bostic: The only lan­guage that looks the same before and after RSA encryption.”

So let’s address, con­front, and demys­ti­fy this hate. What are these objec­tion­able syn­tac­ti­cal, noisy, pos­si­bly encrypt­ed bits? And why does Perl have them?

Regular expressions

Regular expres­sions, or reg­ex­ps, are not unique to Perl. JavaScript has them. Java has them. Python has them as well as anoth­er mod­ule that adds even more fea­tures. It’s hard to find a lan­guage that does­n’t have them, either native­ly or through the use of a library. It’s com­mon to want to search text using some kind of pat­tern, and reg­ex­ps pro­vide a fair­ly stan­dard­ized if terse mini-​language for doing so. There’s even a C‑based library called PCRE, or Perl Compatible Regular Expressions,” enabling many oth­er pieces of soft­ware to embed a reg­exp engine that’s inspired by (though not quite com­pat­i­ble) with Perl’s syntax.

Being itself inspired by Unix tools like grep, sed, and awk, Perl incor­po­rat­ed reg­u­lar expres­sions into the lan­guage as few oth­er lan­guages have, with bind­ing oper­a­tors of =~ and !~ enabling easy match­ing and sub­sti­tu­tions against expres­sions, and pre-​compilation of reg­ex­ps into their own type of val­ue. Perl then added the abil­i­ty to sep­a­rate reg­ex­ps by white­space to improve read­abil­i­ty, use dif­fer­ent delim­iters to avoid the leaning-​toothpick syn­drome of escap­ing slash (/) char­ac­ters with back­slash­es (\), and name your cap­ture groups and back­ref­er­ences when sub­sti­tut­ing or extract­ing strings.

All this is to say that Perl reg­u­lar expres­sions can be some of the most read­able and robust when used to their full poten­tial. Early on this helped cement Perl’s rep­u­ta­tion as a text-​processing pow­er­house, though the core of reg­ex­ps’ suc­cinct syn­tax can result in difficult-​to-​read code. Such inscrutable exam­ples can be found in any lan­guage that imple­ments reg­u­lar expres­sions; at least Perl offers the enhance­ments men­tioned above.

Sigils

Perl has three built-​in data types that enable you to build all oth­er data struc­tures no mat­ter how com­plex. Its vari­able names are always pre­ced­ed by a sig­il, which is just a fan­cy term for a sym­bol or punc­tu­a­tion mark.

  • A scalar con­tains a string of char­ac­ters, a num­ber, or a ref­er­ence to some­thing, and is pre­ced­ed with a $ (dol­lar sign).
  • An array is an ordered list of scalars begin­ning with an ele­ment num­bered 0 and is pre­ced­ed with a @ (at sign). 
  • A hash, or asso­cia­tive array, is an unordered col­lec­tion of scalars indexed by string keys and is pre­ced­ed with a % (per­cent sign).

So vari­able names $look @like %this. Individual ele­ments of arrays or hash­es are scalars, so they $look[0] $like{'this'}. (That’s the first ele­ment of the @look array count­ing from zero, and the ele­ment in the %like hash with a key of 'this'.)

Perl also has a con­cept of slices, or select­ed parts of an array or hash. A slice of an array looks like @this[1, 2, 3], and a slice of a hash looks like @that{'one', 'two', 'three'}. You could write it out long-​hand like ($this[1], $this[2], $this[3]) and ($that{'one'}, $that{'two'}, $that{'three'} but slices are much eas­i­er. Plus you can even spec­i­fy one or more ranges of ele­ments with the .. oper­a­tor, so @this[0 .. 9] would give you the first ten ele­ments of @this, or @this[0 .. 4, 6 .. 9] would give you nine with the one at index 5 miss­ing. Handy, that.

In oth­er words, the sig­il always tells you what you’re going to get. If it’s a sin­gle scalar val­ue, it’s pre­ced­ed with a $; if it’s a list of val­ues, it’s pre­ced­ed with a @; and if it’s a hash of key-​value pairs, it’s pre­ced­ed with a %. You nev­er have to be con­fused about the con­tents of a vari­able because the name will tell you what’s inside.

Data structures, anonymous values, and dereferencing

I men­tioned ear­li­er that you can build com­plex data struc­tures from Perl’s three built-​in data types. Constructing them with­out a lot of inter­me­di­ate vari­ables requires you to use things like:

  • lists, denot­ed between ( paren­the­ses )
  • anony­mous arrays, denot­ed between [ square brack­ets ]
  • and anony­mous hash­es, denot­ed between { curly braces }.

Given these tools you could build, say, a scalar ref­er­enc­ing an array of street address­es, each address being an anony­mous hash:

$addresses = [
  { 'name'    => 'John Doe',
    'address' => '123 Any Street',
    'city'    => 'Anytown',
    'state'   => 'TX',
  },
  { 'name'    => 'Mary Smith',
    'address' => '100 Other Avenue',
    'city'    => 'Whateverville',
    'state'   => 'PA',
  },
];

(The => is just a way to show cor­re­spon­dence between a hash key and its val­ue, and is just a fun­ny way to write a com­ma (,). And like some oth­er pro­gram­ming lan­guages, it’s OK to have trail­ing com­mas in a list as we do for the 'state' entries above; it makes it eas­i­er to add more entries later.)

Although I’ve nice­ly spaced out my exam­ple above, you can imag­ine a less socia­ble devel­op­er might cram every­thing togeth­er with­out any spaces or new­lines. Further, to extract a spe­cif­ic val­ue from this struc­ture this same per­son might write the fol­low­ing, mak­ing you count dol­lar signs one after anoth­er while read­ing right-​to-​left then left-to-right:

say $$addresses[1]{'name'};

We don’t have to do that, though; we can use arrows that look like -> to deref­er­ence our array and hash elements:

say $addresses->[1]->{'name'};

We can even use post­fix deref­er­enc­ing to pull a slice out of this struc­ture, which is just a fan­cy way of say­ing always read­ing left to right”:

say for $addresses->[1]->@{'name', 'city'};

Which prints out:

Mary Smith
Whateverville

Like I said above, the sig­il always tells you what you’re going to get. In this case, we got:

  • a sliced list of val­ues with the keys 'name' and 'city' out of…
  • an anony­mous hash that was itself the sec­ond ele­ment (count­ing from zero, so index of 1) ref­er­enced in…
  • an anony­mous array which was itself ref­er­enced by…
  • the scalar named $addresses.

That’s a mouth­ful, but com­pli­cat­ed data struc­tures often are. That’s why Perl pro­vides a Data Structures Cookbook as the perldsc doc­u­men­ta­tion page, a ref­er­ences tuto­r­i­al as the perlreftut page, and final­ly a detailed guide to ref­er­ences and nest­ed data struc­tures as the perlref page.

Special variables

Perl was also inspired by Unix com­mand shell lan­guages like the Bourne shell (sh) or Bourne-​again shell (bash), so it has many spe­cial vari­able names using punc­tu­a­tion. There’s @_ for the array of argu­ments passed to a sub­rou­tine, $$ for the process num­ber the cur­rent pro­gram is using in the oper­at­ing sys­tem, and so on. Some of these are so com­mon in Perl pro­grams they are writ­ten with­out com­men­tary, but for the oth­ers there is always the English mod­ule, enabling you to sub­sti­tute in friend­ly (or at least more awk-like) names.

With use English; at the top of your pro­gram, you can say:

All of these pre­de­fined vari­ables, punc­tu­a­tion and English names alike, are doc­u­ment­ed on the perlvar doc­u­men­ta­tion page.

The choice to use punc­tu­a­tion vari­ables or their English equiv­a­lents is up to the devel­op­er, and some have more famil­iar­i­ty with and assume their read­ers under­stand the punc­tu­a­tion vari­ety. Other less-​friendly devel­op­ers engage in code golf,” attempt­ing to express their pro­grams in as few key­strokes as possible.

To com­bat these and oth­er unso­cia­ble ten­den­cies, the perlstyle doc­u­men­ta­tion page admon­ish­es, Perl is designed to give you sev­er­al ways to do any­thing, so con­sid­er pick­ing the most read­able one.” Developers can (and should) also use the perlcritic tool and its includ­ed poli­cies to encour­age best prac­tices, such as pro­hibit­ing all but a few com­mon punc­tu­a­tion vari­ables.

Conclusion: Do you still hate Perl?

There are only two kinds of lan­guages: the ones peo­ple com­plain about and the ones nobody uses.

Bjarne Stroustrup, design­er of the C++ pro­gram­ming language

It’s easy to hate what you don’t under­stand. I hope that read­ing this arti­cle has helped you deci­pher some of Perl’s noisy” quirks as well as its fea­tures for increased read­abil­i­ty. Let me know in the com­ments if you’re hav­ing trou­ble grasp­ing any oth­er aspects of the lan­guage or its ecosys­tem, and I’ll do my best to address them in future posts.

person doing card trick

Perl is said (some­times frus­trat­ing­ly) to be a do-​what-​I-​mean pro­gram­ming lan­guage. Many of its state­ments and con­struc­tions are designed to be for­giv­ing or have analo­gies to nat­ur­al lan­guages. Still oth­ers are said to be mag­ic,” behav­ing dif­fer­ent­ly depend­ing on how they’re used. Adept use of Perl asks you to not only under­stand this mag­ic, but to embrace it and the expres­sive­ness it enables. Here, then, are five ways you can bring some mag­ic to your code.

$_

Perl has many spe­cial vari­ables, and first among them (lit­er­al­ly, it’s the first doc­u­ment­ed) is $_. Also spelled $ARG if you use the English mod­ule, the doc­u­men­ta­tion describes it as the default input and pattern-​matching space.” Many, many func­tions and state­ments will assume it as the default or implic­it argu­ment; you can find the full list in the doc­u­men­ta­tion. Here’s an exam­ple that uses it implic­it­ly to out­put the num­bers from 1 to 5:

say for 1 .. 5;

Output:

1
2
3
4
5

Where some lan­guages require an iter­a­tor vari­able in a for or foreach loop, in the absence of one Perl assigns it to $_.

Statement modifiers

We then use our sec­ond trick; where some oth­er lan­guages require a block to enclose every loop or con­di­tion­al (whether denot­ed by braces { } or inden­ta­tion), Perl allows you to put said loop­ing or con­di­tion­al state­ment after a sin­gle oth­er state­ment, in this case the say which prints its argument(s) fol­lowed by a newline.

However, above we have no argu­ments passed to say and so once again the default $_ is used, now con­tain­ing a num­ber from 1 to 5 which is then print­ed out. It’s a very pow­er­ful and expres­sive idiom, enabling both the writer and read­er of code to con­cen­trate on the impor­tant thing that’s hap­pen­ing. It’s also entire­ly option­al. You can just as eas­i­ly type:

for my $foo (1..5) {
    say $foo;
}

But where’s the mag­ic in that?

Magic variables and use English

We men­tioned the $_ vari­able above, and that it could also be spelled $ARG if you add use English to your code. It can be hard to read code with large amounts of punc­tu­a­tion, though, and even hard­er to remem­ber what each vari­able does. Thankfully the English mod­ule pro­vides alias­es, and the per­l­var man page lists them in order. It’s much eas­i­er to read and write things like $LIST_SEPARATOR, $PROCESS_ID, or $MATCH rather than $", $$, and $&, and goes a long way towards reduc­ing Perl’s rep­u­ta­tion as a write-​only language.

List and scalar contexts

Like nat­ur­al lan­guages, Perl has a con­cept of con­text” in which words mean dif­fer­ent things depend­ing on their sur­round­ings. In Perl’s case, expres­sions may behave dif­fer­ent­ly depend­ing on whether they expect to pro­duce a list of val­ues or a sin­gle val­ue, called a scalar. Here’s a triv­ial example:

my @foo = (1, 2, 3); # list context, @foo contains the list
my $bar = (1, 2, 3); # scalar context, $bar contains 3

In the first line, we assign the list of num­bers (1, 2, 3) to the array @foo. But in the sec­ond line, we’re assign­ing to the scalar vari­able $bar, which now con­tains the last item in the list.

Here’s anoth­er exam­ple, using the reverse function:

my @foo = ('one', 'two', 'three');
my @bar = reverse @foo; # @bar contains ('three', 'two', 'one')
my $baz = reverse @foo; # $baz contains 'eerhtowteno'

In list con­text, reverse takes its argu­ments and returns them in the oppo­site order. But in scalar con­text, it con­cate­nates all of the argu­ments togeth­er and returns a string with the char­ac­ters in oppo­site order.

In gen­er­al, there is no gen­er­al rule for deduc­ing a func­tion’s behav­ior in scalar con­text from its behav­ior in list con­text.” (Dominus 1998) You’ll just have to look up the func­tion to deter­mine what it does, though in gen­er­al, it does what you want, but if you want to force scalar con­text use the scalar operator:

my @foo = ('aa', 'aab', 'bbc');
my @bar = scalar grep /aa/, @foo; # returns a list (2), counting the number of matches

Hash slices

One of Perl’s three built-​in data types is the hash, also known as an asso­cia­tive array. It’s an unordered col­lec­tion of scalars indexed by string, rather than the num­bers used by nor­mal arrays. It’s a use­ful con­struct, and you can devel­op com­pli­cat­ed data struc­tures using just scalars, arrays, and hash­es. What’s not wide­ly known is that you can access sev­er­al ele­ments of of a hash using a hash slice, using syn­tax that’s sim­i­lar to array slices. Here’s an example:

my ($who, $home) = @ENV{'USER', 'HOME'};

It works the oth­er way, too: you can assign to a slice.

@colors{'red', 'green', 'blue'} = (0xff0000, 0x00ff00, 0x0000ff);

I use this a lot when assign­ing argu­ments received from func­tions or meth­ods (see my pre­vi­ous arti­cle on sub­rou­tine sig­na­tures):

use v5.24; # for postfix dereferencing
use Types::Standard qw(Str Int);
use Type::Params 'compile_named';

foo('hello', 42);

sub foo {
    state $check = compile_named(
        param1 => Str,
        param2 => Int, {optional => 1},
    );
    my ($param1, $param2) =
        $check->(@_)->@{'param1', 'param2'};

    say $param1, $param2;
}

In the exam­ple above, $check->(@_) returns the type-​checked argu­ments to the foo() func­tion cour­tesy of Type::Paramscompile_named() func­tion. It’s returned as a hash ref­er­ence, and since hash­es are unordered, we spec­i­fy the order in which we want the val­ues by deref­er­enc­ing and then slic­ing the result­ing hash. The post­fix deref­er­enc­ing syn­tax was added in Perl 5.20 and made a default fea­ture in 5.24, and reduces the num­ber of nest­ed brack­ets and braces we have to deal with.

Conclusion

I hope this arti­cle has giv­en you a taste of some of the mag­ic avail­able in the Perl lan­guage. It’s these sort of fea­tures that make pro­gram­ming in it a bit more joy­ful. As always, check the doc­u­men­ta­tion for com­plete infor­ma­tion on these and oth­er top­ics, or look for answers and ask ques­tions on PerlMonks or Stack Overflow.