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.

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