I remem­ber a brief time in the mid-​2000s insist­ing on so-​called Yoda con­di­tions” in my Perl. I would place con­stants to the left of equal­i­ty com­par­isons. In case I acci­den­tal­ly typed a sin­gle = instead of ==, the com­pil­er would catch it instead of blithe­ly assign­ing a vari­able. E.g.:

if ( $foo == 42 ) { ... } # don’t do this
if ( 42 == $foo ) { ... } # do this
if ( $foo = 42  ) { ... } # to prevent this

And because a fool­ish con­sis­ten­cy is the hob­gob­lin of lit­tle minds, I would even extend this to string and rela­tion­al comparisons.

if ( 'bar' eq $foo ) { ... } # weirdo
if ( 42 > $foo )     { ... } # make it stop

It looks weird, and it turns out it’s unnec­es­sary as long as you pre­cede your code with use warnings;. Perl will then warn you: Found = in conditional, should be ==“. (Sidenote: Perl v5.36, due in mid-​2022, is slat­ed to enable warn­ings by default if you do use v5.35; or above, in addi­tion to the strict­ness that was enabled with use v5.11;. Yay for less boilerplate!)

If you want to fatal­ly catch this and many oth­er warn­ings, use the stric­tures mod­ule from CPAN in your code like this:

use strictures 2;

This will cause your code to throw an excep­tion if it com­mits many cat­e­gories of mis­takes. If you’re run­ning in a ver­sion con­trol sys­tem’s work­ing direc­to­ry (specif­i­cal­ly Git, Subversion, Mercurial, or Bazaar), the mod­ule also pre­vents you from using indi­rect object syn­tax, Perl 4‑style mul­ti­di­men­sion­al arrays, and bare­word file­han­dles.

Getting back to assign­ments vs. con­di­tion­als, there is one case where I’ve found it to be accept­able to use an assign­ment inside an if state­ment, and that’s when I need to use the result of a check inside the con­di­tion. For example:

if ( my $foo = some_truthy_function() ) {
    ... # do something further with $foo

This keeps the scope of some_truthy_function()s result inside the block so that I don’t pol­lute the out­er scope with a tem­po­rary vari­able. Fortunately, Perl does­n’t warn on this syntax.

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.

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.


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

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.

Back To The Future DeLorean

Last week saw the release of Perl 5.34.0 (you can get it here), and with it comes a year’s worth of new fea­tures, per­for­mance enhance­ments, bug fix­es, and oth­er improve­ments. It seems like a good time to high­light some of my favorite changes over the past decade and a half, espe­cial­ly for those with more dat­ed knowl­edge of Perl. You can always click on the head­ers below for the full releas­es’ perldelta pages.

Perl 5.10 (2007)

This was a big release, com­ing as it did over five years after the pre­vi­ous major 5.8 release. Not that Perl devel­op­ers were idle—but it would­n’t be until ver­sion 5.14 that the lan­guage would adopt a steady year­ly release cadence.

Due to the build-​up time, many core enhance­ments were made but the most impor­tant was arguably the feature prag­ma, enabling the addi­tion of new syn­tax that would oth­er­wise break Perl’s back­ward com­pat­i­bil­i­ty. 5.10 also intro­duced the defined-​or oper­a­tor (//), state vari­ables that per­sist their pre­vi­ous val­ue, the say func­tion for auto­mat­i­cal­ly append­ing a new­line on out­put (so much saved typ­ing), and a large col­lec­tion of improve­ments to reg­u­lar expres­sions. In addi­tion, this release intro­duced smart match­ing (~~), though ver­sion 5.18 would even­tu­al­ly rel­e­gate it to exper­i­men­tal sta­tus.

Perl 5.12 (2010)

This release also saw many new fea­tures added, but if I had to pick one mar­quee item it would be exper­i­men­tal sup­port for plug­gable key­words, which enabled authors to extend the lan­guage itself with­out mod­i­fy­ing the core. Previously one would either use plain func­tions, hacky source fil­ters, or the dep­re­cat­ed Devel::Declare mod­ule to sim­u­late this func­tion­al­i­ty. CPAN authors would go on to cre­ate all kinds of new syn­tax, some­times pro­to­typ­ing fea­tures that would even­tu­al­ly make their way into core.

Perl 5.14 (2011)

5.14 had a big list of enhance­ments, includ­ing Unicode 6.0 sup­port and a gag­gle of reg­u­lar expres­sion fea­tures. My favorite of these was the /r switch for non-​destructive sub­sti­tu­tions.

But as the first year­ly cadence release, the changes in pol­i­cy took cen­ter stage. The Perl 5 Porters (p5p) explic­it­ly com­mit­ted to sup­port­ing the two most recent sta­ble release series, pro­vid­ing secu­ri­ty patch­es only for release series occur­ring in the past three years. They also defined an explic­it com­pat­i­bil­i­ty and dep­re­ca­tion pol­i­cy, with def­i­n­i­tions for fea­tures that may be exper­i­men­tal, dep­re­cat­ed, dis­cour­aged, and removed.

Perl 5.16 (2012)

Another year, anoth­er ver­sion bump. This time the core enhance­ments were all over the map (although no enhance­ments to the map function 😀 ).

May I high­light anoth­er doc­u­men­ta­tion change, though? The perlootut Object-​Oriented Programming in Perl Tutorial replaced the old perltoot, perltooc, perlboot, and perlbot pages, pro­vid­ing an intro­duc­tion to object-​oriented design con­cepts before strong­ly rec­om­mend­ing the use of one of the OO sys­tems from CPAN. Mentioned are Moose, its alter­na­tive Mouse, Class::Accessor, Object::Tiny, and Role::Tinys usage with the lat­ter two. Later ver­sions of perlootut would rec­om­mend Moo rather than Mouse.

Perl 5.18 (2013)

As men­tioned ear­li­er, Perl 5.18 ren­dered smart­match exper­i­men­tal, as well as lex­i­cal use of the $_ vari­able. With these came a new cat­e­go­ry of warn­ings for exper­i­men­tal fea­tures and a method for over­rid­ing such warn­ings feature-​by-​feature. Fitting in with the secu­ri­ty and safe­ty theme, hash­es were over­hauled to ran­dom­ize key/​value order, increas­ing their resis­tance to algo­rith­mic com­plex­i­ty attacks.

But it was­n’t all fenc­ing in bad behav­ior. Lexical sub­rou­tines made their first (exper­i­men­tal) appear­ance, and although I con­fess I haven’t had much call for them in my work, oth­ers have come up with some inter­est­ing uses. Four years lat­er they became non-​experimental.

Perl 5.20 (2014)

Three new syn­tax fea­tures arrived in 2014: exper­i­men­tal sub­rou­tine sig­na­tures (of which I’ve writ­ten more about here), key/​value hash slices and index/​value array slices, and exper­i­men­tal post­fix deref­er­enc­ing. This last enables clean­er left-​to-​right syn­tax when deref­er­enc­ing variables:

  • @{ $array_ref } becomes $array_ref->@*
  • %{ $hash_ref } becomes $hash_ref->%*
  • Etc.

Postfix deref­er­enc­ing became non-​experimental in Perl 5.24, and vig­or­ous dis­cus­sion con­tin­ues on sub­rou­tine sig­na­tures’ future.

Perl 5.22 (2015)

Speaking of sub­rou­tine sig­na­tures, their loca­tion moved to between the sub­rou­tine name (if any) and the attribute list (if any). Previously they appeared after attrib­ut­es. The les­son? Remain con­scious of exper­i­men­tal fea­tures in your code, and be pre­pared to make changes when upgrading.

In addi­tion to the enhance­ments, secu­ri­ty updates, per­for­mance fix­es, and dep­re­ca­tions, devel­op­ers removed the his­tor­i­cal­ly notable CGI mod­ule. First added to core in 1997 in recog­ni­tion of its crit­i­cal role in enabling web devel­op­ment, it’s been sup­plant­ed by bet­ter alter­na­tives on CPAN.

Perl 5.24 (2016)

Perl 5.20s post­fix deref­er­enc­ing was no longer exper­i­men­tal, and devel­op­ers removed both lex­i­cal $_ and autoderef­er­enc­ing on calls to push, pop, shift, unshift, splice, keys, values, and each.

Perl 5.26 (2017)

The incor­po­ra­tion of exper­i­men­tal fea­tures con­tin­ued, with lex­i­cal sub­rou­tines mov­ing into full sup­port. I like the added read­abil­i­ty enhance­ments, though: indent­ed here-​documents; the /xx reg­u­lar expres­sion mod­i­fi­er for tabs and spaces in char­ac­ter class­es; and @{^CAPTURE}, %{^CAPTURE}, and %{^CAPTURE_ALL} for reg­exp match­es with a lit­tle more self-documentation.

Perl 5.28 (2018)

Experimental sub­rou­tine sig­na­ture and attribute order­ing flipped back to its Perl 5.20 sequence of attributes-​then-​signature. Bit of a roller­coast­er ride on this one. You could do worse than using some­thing like Type::Params until this set­tles and get a wide vari­ety of type con­straints in the bargain.

Perl 5.30 (2019)

Pour one out for AWK and Fortran pro­gram­mers migrat­ing to Perl: the $[vari­able for set­ting the low­er bound of arrays could no longer be set to any­thing oth­er than zero. This had a long dep­re­ca­tion cycle start­ing in Perl 5.12.

Perl 5.32 (2020)

In 2020 Perl’s devel­op­ment moved to GitHub. And once again, I’m going to high­light read­abil­i­ty enhance­ments: the exper­i­men­tal isa oper­a­tor could be used to say:

if ( $obj isa Some::Class ) { ... }

instead of

use Scalar::Util 'blessed';
if ( blessed($obj) and $obj->isa('Some::Class') { ... }

You could also chain com­par­i­son oper­a­tors, lead­ing to the more math­e­mat­i­cal­ly con­cise if ( $x < $y <= $z ) {...} rather than if ( $x < $y and $y <= $z ) {...}.

Perl 5.34 (2021)

Finally, we come to last week’s release and its intro­duc­tion of exper­i­men­tal try/​catch excep­tion han­dling syn­tax. If you need to sup­port ear­li­er ver­sions of Perl back to 5.14, you can use Feature::Compat::Try. Earlier this year I inter­viewed the fea­ture and mod­ule’s author, Paul LeoNerd” Evans, for Perl.com. This year also marked the debut of Perl’s new gov­er­nance mod­el with the appoint­ment of a Core Team and a three-​member Steering Council.

What are some of your favorite Perl improve­ments over the years? Check out the perlhist doc­u­ment for a detailed chronol­o­gy and refresh­er with the var­i­ous perldelta pages and leave me a com­ment below.

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;



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.


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.