Get started with Perl 6 one liners

One thing that sets Perl apart from other languages is the ability to write small programs in a single line of code, known as a “one liner”. It’s often faster to type a program directly into the terminal than to write a throwaway script. And one liners are powerful too; they’re complete Perl programs that can load external libraries but also integrate into the terminal. You can pipe data in or out of a one liner.

Like its predecessor, Perl 6 supports one liners. And in the same way Perl 6 cleaned up Perl 5’s warts elsewhere, the one liner syntax is also better. It’s cleaner with fewer special variables and options to memorize. This article aims to get you up-and-running with Perl 6 one liners.

The basics

To get started with one liners, all you really need to understand is the -e option. This tells Perl to execute what follows as a program. For example:

perl6 -e 'say "Hello, World!"'

Let’s step through this code.

  1. perl6 invokes the Perl 6 program
  2. -e tells Perl 6 to execute
  3. 'say "Hello, World!"' is the program. Every program must be surrounded in single quotes (except on Windows, see (converting for Windows).

To run a one-liner, just type it into the terminal:

> perl6 -e 'say "Hello, World!"'
Hello, World!

File processing

If you want to load a file, just add the path to the file after the program code:

> perl6 -e 'for (lines) { say $_ }' /path/to/file.txt

This program prints every line in /path/to/file.txt. You may know that $_ is the default variable, which in this case is the current line being looped through. lines is a list that is automatically created for you whenever you pass a filepath to a one-liner. Now let’s re-write that one liner, step-by-step. These one liners are all equivalent:

> perl6 -e 'for (lines) { say $_ }' /path/to/file.txt
> perl6 -e 'for (lines) { $_.say }' /path/to/file.txt
> perl6 -e 'for (lines) { .say }' /path/to/file.txt
> perl6 -e '.say for (lines)' /path/to/file.txt
> perl6 -e '.say for lines' /path/to/file.txt

Just like $_ is the default variable, methods called on the default variable can omit the variable name. They become default methods. So $_.say becomes .say. This brevity pays off with one liners - it’s less typing!

The -n option changes the behavior of the program: it executes the code once for every line of the file. To uppercase and print every line of /path/to/file.txt you can type:

> perl6 -ne '.uc.say' /path/to/file.txt

The -p option is just like -n except that it will automatically print $_. This means that another way we could uppercase a file would be:

> perl6 -pe '$_ = $_.uc' /path/to/file.txt

Or by applying a shortcut, this does the same thing:

> perl6 -pe '.=uc' /path/to/file.txt

The -n and -p options are really useful and often spare the programmer from extra typing.

Load modules

The final thing you should know is how to load a module. This is really powerful as you can extend Perl 6’s capabilities by importing external libraries. The -M switch stands for load module:

> perl6 -M URI::Encode -e 'say encode_uri("/10 ways to crush it with Perl 6")'

The code -M URI::Encode loads the URI::Encode module, which exports the encode_uri subroutine. It prints:


What if you have a module that is not installed in a standard location? In this case using -M alone won’t work, as Perl won’t find the module. For these scenarios, just pass use the -I switch to include the directory:

> perl6 -I lib -M URI::Encode -e 'say encode_uri(" ways to crush it with Perl 6")'

Now Perl 6 will search for URI::Encode in lib as well as the standard install locations.

Finally, if you want a summary of all of these options, just use the -h option:

> perl6 -h

This will print:

    With no arguments, enters a REPL. With a "[programfile]" or the "-e" option, compiles the given program and by default also executes the compiled code.
    -c                   check syntax only (runs BEGIN and CHECK blocks)
    --doc                extract documentation and print it as text
    -e program           one line of program
    -h, --help           display this help text
    -n                   run program once for each line of input
    -p                   same as -n, but also print $_ at the end of lines
    -I path              adds the path to the module search path
    -M module            loads the module prior to running the program
    --target=[stage]     specify compilation stage to emit
    --optimize=[level]   use the given level of optimization (0..3)
    -t, --trace=[flags]  enable trace flags, see 'parrot --help-debug'
    --encoding=[mode]    specify string encoding mode
    -o, --output=[name]  specify name of output file
    -v, --version        display version information
    --stagestats         display time spent in the compilation stages
    --ll-exception       display a low level backtrace on errors
    --profile            print profile information to standard error
    --doc=[module]       Use Pod::To::[module] to render inline documentation.
    Note that only boolean single-letter options may be bundled.

    Output from --profile can be visualized by kcachegrind.

    To modify the include path, you can set the PERL6LIB environment variable:

    PERL6LIB="lib" perl6

    For more information, see the perl6(1) man page.


This article was adapted from my open source book, which has lots of example Perl 6 one liners, many of which were contributed by the Perl 6 community. If you’re interested in learning more Perl 6, I’d recommend visiting the official website, which has links to the IRC channel and official documentation.


David Farrell

David is the founder and editor of An organizer of the New York Perl Meetup, he works as a technology consultant in New York City.

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