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If you've discovered something amazing about Perl that you just need to share with everyone, this is the right place.

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Meditations is sometimes used as a sounding-board — a place to post initial drafts of perl tutorials, code modules, book reviews, articles, quizzes, etc. — so that the author can benefit from the collective insight of the monks before publishing the finished item to its proper place (be it Tutorials, Cool Uses for Perl, Reviews, or whatever). If you do this, it is generally considered appropriate to prefix your node title with "RFC:" (for "request for comments").

User Meditations
I love Perl and I love Mojolicious
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by coetry
on Jul 31, 2017 at 00:53
    I haven't done any web development in perl before today and I started with Mojolicious. I've worked with Rails apps before and that's where I learn about models, views, controllers, routes, etc. I understood what happens when a user sends a request from the client and how that request gets handled and served back by the browser. The Ruby ecosystem has a ton of gems (libraries) specifically tailored towards Rails and its definitely a huge full featured framework sort of like how Perl is such a full featured language. I've also actually played with other ruby web frameworks like Sinatra. I'm able to say with my experience with other languages and frameworks that web development with Perl and Mojolicious is very fun. I'm new to Perl in general but have fallen in love with its free-form syntax, plethora of operators, expressiveness, clever community, and of course the documentation is unparalleled to anything I've seen elsewhere. The fact that Mojolicious makes it so easy to expose our crafty Perl hacks from our terminals to the web in such an organized, clean, simple, and fun manner truly makes me smile. I do know that Mojolicious isn't the only option out there to do this. I've heard great thinks about Dancer2 and other options as well. I'm not sure what I'll end up building, but maybe I'll share snippets of that journey in due time!
Perl Security Testing
4 direct replies — Read more / Contribute
by zentara
on Jul 24, 2017 at 10:02
    Hi, the Test Driven Development, for software and for pancakes node interested me, and I went off on a tangent from talexb's original meditation. So I post a new meditation, with my reply as a starter.

    Original reply: ##########################

    I'm a total amateur compared to you fellows, but I do find when I write my code, for the first draft, I almost always print out arrays and variables after everytime I use them. I almost always get things wrong the first time thru, so my method is very helpful to me.

    It's my guess is that the reason TDD failed is that the Test that you didn't account for, is the one that causes the bug, ( if any).

    What is more worring to me is the security vulnerabilities which Perl5 is susceptible to.

    For instance, could a normal or guest user on your machine, with access to Perl scripts, cause a buffer-overflow of some sort, and gain root access? I'm sure the NSA would pay for that information. :-)

    How safe is Perl out there in the wild? Are systems being hacked thru Perl? As far as know, Perl has been very safe in my limited use. I guess security is the number one test.

    So what do you experts feel, know, and or are hiding concerning Perl's security, assuming the scripts are written and run correctly? Was there ever a real buffer overflow exploit? etc

    Should I worry about other users on my linux box getting root escalation if I let them login?

    I'm not really a human, but I play one on earth. ..... an animated JAPH
Test Driven Development, for software and for pancakes
3 direct replies — Read more / Contribute
by talexb
on Jul 23, 2017 at 11:16

    It's Sunday morning in Toronto, so that means pancakes for breakfast.

    I use the same recipe from the Joy of Cooking that I've used for the last fifteen years or so, which starts with me heating up my two cast iron pans to a 4.5 setting. After making the batter, I make just one pancake in the middle of each pan, and check that they cook at the right speed to confirm I have the correct pan temperature. And I taste the finished pancake, to make sure I haven't messed up the recipe. (Yes, it's possible to forget a crucial ingredient when following a well-worn recipe -- it's amazing what forgetting salt does do a dish.)

    I make pancakes four to a pan, so they're 'silver dollar' pancakes, not more than three inches across. I monitor the colour and the speed at which they're cooking as I go, adjusting the heat if necessary. Finished pancakes go into a covered dish to stay warm.

    I believe I'm a careful cook, and I'm also a careful developer. That means test early, and test often. You want no surprises when it's time to dish up the goods to your family (pancakes) or your customers (software). Dreaming up new dishes or new applications it's important to be creative, to try new approaches. Cooking or writing software, you have to be methodical, thoughtful, and meticulous.

    I consider myself very lucky to have gone to a really good university, but what I studied (Systems Design Engineering) bears only a tangential relationship to the work I do now. It was actually really good training at reading/comprehension, time management, mental gymnastics, and getting the job done. As long as another program had the same goals, I think pretty much any degree or diploma would have done. An Engineering degree from Waterloo just helped me get my foot in the door that much easier.

    Conclusion? Do your development with tests, right from the start. You'll thank yourself, again and again. And it makes for good pancakes. ;)

    Alex / talexb / Toronto

    Thanks PJ. We owe you so much. Groklaw -- RIP -- 2003 to 2013.

Perl, JavaScript and Strandbeests
2 direct replies — Read more / Contribute
by cavac
on Jul 20, 2017 at 08:19

    A little over a year ago, i have been playing with AI::Genetic, Javascript::V8 and Strandbeest evolution for a Linuxdays Demo. I wrote this originally as a distributed genetic evolution demo, complete with managment web interface, but for the sake of clarity, here is the single-user command-line version of it.

    (I might do another meditation later that show more of the distributed stuff, but my Raspberry Pi cluster is still in a packing crate in the attic... i hope... after moving house in a chaotioc fashion).

    Before i begin, you should note that the Javascript::V8 version on CPAN has a memory leak. You might try my own, non-official version at It is in a mercurial repo, so you should be able to do this:

    hg clone cd JavaScript-V8 perl Makefile.PL make test install distclean

    The magic of Strandbeests

    I was always fascinated my Strandbeests, the huge moving "animals" created by Theo Jansen. Although mostly made out of PVC tubes, they exhibit a really strange, lifelike movement.

    The key part of this creatures of the length relationship between their eleven leg parts that lets them walk with very little effort. As Theo put it: "But even for the computer the number of possible ratios between 11 rods was immense. Suppose every rod can have 10 different lengths, then there are 10,000,000,000,000 possible curves. If the computer were to go through all these possibilities systematically, it would be kept busy for 100,000 years. I didn't have this much time, which is why I opted for the evolutionary method."

    Computers? Doing evolution? Hell yeah, i needed to learn how thats done!

    Why JavaScript?

    While i'm a competent developer when it comes to PostgreSQL databases, HTML and similar stuff, a lot of the math required for analyzing and evaluating the movement of a Strandbeest leg is completely outside my comfort zone (i.e. i have absolutely no clue how to do it).

    Thankfully, someone on Stackoverflow made a nice Javascript demo, which (for my Linuxdays thing) also solved the HTML visualization problem for me.

    Perl to the rescue

    This is where i came in. Now i had two options. Either convert the JS code to Perl (and keep it compatible with the JS version, so the HTML visu was in sync), or just use Javascript without the drawing functions from within Perl. I chose the latter, because it seemed easier.

    To run the genetic evolution algorithm itself, i chose the AI::Genetic module, since this seemed the easiest interface for what i needed.

    Before we begin, we need to define (which is a simplified copy from the one in my maplat_helpers repo):

    We also need James Coglan's Sylvester Vector and Matrix library for JavaScript, saved as "sylvester.js"

    And last but not least in our list of "support" files, the Strandbeest evaluation function itself, saved in a file with the name "strandbeest.js". This is the one with all the graphical functions removed:

    Now that we have to copied portions of our code, let's write some of our own.

    This is basically our caller for the main module and runs the main loop. First we check for some command line arguments, set up the evolver, and then run in an infinite loop.

    We want to support 4 different arguments:

    • --load Loads previous best 3 fits from fittest.dat and seeds the population with them.
    • --save Every time we find a new best fit, we save the best 3 members of the population to fittest.dat.
    • --population_size=1000 Set how many members does each generation have.
    • --crosspolination_count=10 Set the level of cross-polination during evolution.

    Here is the code (file ""):

    Most of what does, is a pretty straight forward wrapper around AI::Genetic, except the Javascript-Handling:

    use JavaScript::V8; ... my $vectorjs = slurpBinFile('sylvester.js'); my $strandbeestjs = slurpBinFile('strandbeest.js'); my $js = $vectorjs . ' ' . $strandbeestjs; ... my $ok = -1; my $errortype = 'MATH_ERROR'; my %scores; my $ctx = JavaScript::V8::Context->new(); $ctx->bind_function(write => sub { print @_ }); $ctx->bind_function(setFailed => sub { $ok = 0; $errortype = shift @_; }); $ctx->bind_function(setFinished => sub { $ok = 1; %scores = @_; }); $ctx->bind(params => \%params); $ctx->eval($js); my $error = $@; my $total = -1000; # Default: Failed! if(defined($error)) { print("SCRIPT ERROR: $error\n"); } elsif($ok == -1) { print("Script didn't call setFailed() or setFinished()\n"); } ...

    Here is the full code of

    While i'm a bit sparse here with explanations (never was the type who could do the teaching stuff), it should give you a nice overview of both how to run Genetic algorithms in Perl, as well as how to use existing JavaScript code to solve a problem.

    Additional information

Fun with Prototypes
4 direct replies — Read more / Contribute
by haukex
on Jul 19, 2017 at 07:54

    Can you tell me:

    1. How does this get parsed? (e.g. try adding parentheses to show precedence)
    2. Given that the body of each of the four subs is { print Dumper(\@_) }, what is the output?
    $_ = "#x"; my @a = qw/b c/; foo /#(\w+)/, @a; bar /#(\w+)/, @a; quz /#(\w+)/, @a; baz /#(\w+)/, @a;



Error reporting in Perl
6 direct replies — Read more / Contribute
by Jambo Hamon
on Jul 12, 2017 at 20:41
    Use of /c modifier is meaningless without /g at (eval 14) line 1, <__A +NONIO__> line 4.

    Would it not be nice to see the offending string in an error code, taint approved? And if it's not a string... Then it's not Perl.

    I slay me,

Barstool Chatter
3 direct replies — Read more / Contribute
by Jambo Hamon
on Jul 09, 2017 at 21:00
    What will the pundits say Perl can't do next?
    $pundits = shift
Goedel's Proof of God in Perl
2 direct replies — Read more / Contribute
by Jambo Hamon
on Jul 09, 2017 at 08:38
A meditation on the naming of perl6
4 direct replies — Read more / Contribute
by stevieb
on Jul 07, 2017 at 09:38
3 direct replies — Read more / Contribute
by Jambo Hamon
on Jul 06, 2017 at 21:21


    How many meditations does it take to reach ``Knowledge of Self''?

    The following links to the randomly choosing elements from an array node. It is meant to show the reflective nature of $_ and is not an excercise in using or creating a link. Display purposes only. :] +&uact=8&ved=0ahUKEwj0yqChgPbUAhVBeD4KHXnPDJsQFggwMAE&url=http%3A%2F%2 +0from%2520an%2520array&usg=AFQjCNEYUua_3UVPyzYBRG4ruf_ZfYRTXw

    If you squint a bit you can see the GET string looks a lot like Perl code?

    a) It's a string with lot's of punctuation.

    b) It's lexical in nature, meaning it's value is only apparent inside the context of the page "url", coincidence? :]



    I thought one of the purposes of the meditations page was to have meditations.

    I mean that I could try to explain what I was thinking when i though let's paste this link from google as a meditation, but isn't it better to just meditate on it?

    Having fun if you are,

Mahalo - Thank you in Hawaii
1 direct reply — Read more / Contribute
by Jambo Hamon
on Jul 06, 2017 at 19:00
    I just want to say something publicly; Even though it might be taken a million different ways. I want to say thank you to the culture of Perl for engaging me. Culture has many different contexts, and the many different abilities of Perl is what makes it work.
Contemplating a documentation series for my Raspberry Pi work
5 direct replies — Read more / Contribute
by stevieb
on Jul 03, 2017 at 13:53

    So, after over a year of learning, development, implementation and testing, I am very near the point where my Raspberry Pi Perl software is ready for prime-time use by the masses. After a few more tweaks, test additions, documentation updates and one last once-over, I plan on writing a "Howto Pi with Perl" type tutorial, across several articles in a series.

    Trying to put together some sort of sane format, I'm looking to those who have publishing experience or technical writers to give my initial thoughts a once-over and provide me any feedback. Most, if not all portions will include breadboard diagrams of each task, code (of course), pictures if reasonable and in some cases potentially even video clips.

    Initial, broad-view structure:

    • very brief intro to the Pi, its capabilities, and overview of the features/limitations of the software
    • a synopsis of what will be being taught, formats/conventions that will be used
    • basic intro on creating and using a Pi object
    • intro to the most basic part of the Pi, the GPIO
    • examples of creating and using a GPIO pin at the most basic level (INPUT, OUTPUT, HIGH and LOW etc)
    • intro and examples using advanced pin features (interrupts, PWM, pull up/down etc)
    • workshop: using simple pin-based sensors and devices (LEDs, hygrometers, thermo-resistors, distance sensors etc)
    • detailed overview of the communication mechanisms included (I2C, Serial, SPI etc)
    • workshop: detailed overview of using the comms, starting very basic, leading into writing your own device/sensor modules using the software
    • workshop: using an LCD and/or OLED screen to display things on it
    • workshop: intro to and using Analog to Digital converters
    • workshop: intro to and using Digital to Analog converters
    • workshop: intro to and using Digital Potentiometers
    • workshop: intro to and using shift registers (basic through to advanced usage)
    • workshop: using a GPS module
    • workshop: advanced uses: utilizing several ICs and sensors in much larger and more involved projects (probably put together a few of these)
    • workshop: connecting to and using features of an Arduino with the Pi (over varying communication methods, including example Arduino sketches)
    • how this software came about, and some basic information on the underlying parts that allow it to work
    • extensive article on how I ensure the code is fully tested after each commit on a custom hardware test platform
    • possibly some details/photos on some of my real life projects I use at home (my indoor grow room automation system for example)
    • possibly a demo example of writing a web UI with jQuery to automate and provide a GUI of a basic project

    update: I think I'm going to use this as my initial template, so I may periodically update it with feedback and new ideas.../update

    For each element that contains a specific topic, include, where feasible/available a section at the bottom of the doc for advanced users perhaps in <small> or equivalent tags (these are just thoughts, nothing firm):

    • link to datasheet
    • list of issues or possible enhancements
    • link to relevant section in the Pi datasheet
    • link to the specific module that houses the code
    • link to a C/C++ example
    • link to Python equivalent

    Any thoughts on this general first thoughts layout? Anything that I should include that's not already? Anything I should remove? How about any thoughts on the individual elements and the manner in which to present them?

    I think I will create the actual demonstration parts first, then weave in the commentary thereafter. At first thought, this seems like the simplest approach that'll allow me to review all of the actual working parts as I go back through and edit in commentary.



Web Crawling using Perl
2 direct replies — Read more / Contribute
by ckj
on Jun 24, 2017 at 08:49
    Here, I'm going to discuss all the steps to do a web crawling using any language or technology. Crawler/Scrapper/Spider/Bot/ multiple synonyms for same stuff which is basically meant to copy content from any site. Based on website crawlers need to be configured. Please share your feedback, if it's good then I can post it on a larger platform as well where I'm planning to discuss crawling in java etc as well:

    1. Static Content Crawling: Static content are those which are generated once and doesn’t keeps on changing on their own, they need manual intervention and update to push any changes in content. So, such pages are easy to create but such pages are highly prone to security and their content can be crawled easily. So, in this case your crawler would be very efficient and providing accurate result as they simply have to access a page and get details.

    2. Dynamic Content Crawling: Dynamic content are those which keeps changing dynamically, in this case page contains server-side code which allows the server to generate a unique content whenever the page is loaded. Since, these contents are dynamically generated using different technology hence they are very secure as well as very difficult to crawl. So, in this case the crawler creation would be very difficult and getting accurate data is also not possible as class names etc are generated on load and there is no html code available on page to get that. Hence, you may have to use different functionalities here such as using proxy or creating robot.txt.

    Static Content Crawling
    my $mech = WWW::Mechanize->new(); my $response = $mech->get(URL); if ($response->is_success) { print $mech->content; } else { die $response->status_line; }
    Dynamic Content Crawling
    use WWW::mechanize::Firefox; use Data::Dumper; $mech= WWW::Mechanize::Firefox->new(); $mech->get(URL); %arr_ref = (AL => [1795, 1276, 795, 1719, 1363, 1145, 961, 17, 18, 199 +5, 977, 1910, 1691, 21, 1660, 1768], AK => [1145, 961, 1995, 977, 1781, 1704], AZ => [1873, 872, 1145, 690, 1162, 961, 918, 528, 811, 704, 529, 1983, + 931, 40, 1995, 977, 597, 1157, 530, 598, 886, 782, 42, 691, 1945]); foreach my $key (sort keys %arr_ref) { print "$key :: @{$arr_ref{$key}} \n"; $mech->field( stateUSAId => $key ); foreach (@{$arr_ref{$key}}) { $mech->field(institutionUSAId=>$_); print $mech->content; } }
Tkx - bind - append binding
1 direct reply — Read more / Contribute
by kcott
on Jun 23, 2017 at 16:23

    I'm currently working on a module which uses Tkx. I came across something tricky involving appending bindings. It took a while to work out and, in the process, I ran a lot of tests: the results were quite surprising. I've posted this here in case anyone else finds themselves in a similar position: hopefully, it might save them some time and effort.

    Tkx is a thin wrapper around Tcl. Its documentation is minimal: it just links to the Tcl documentation and leaves you to work out how to use it. In this instance, I was looking at the bind command documentation for information on appending a binding. I've linked to all of it, here's the relevant parts for this specific post:


    bind — Arrange for X events to invoke Tcl scripts


    bind tag ?sequence? ?+??script?


    ... If script is prefixed with a "+", then it is appended to any existing binding for sequence; ...

    That's all it says about appending bindings. I investigated this; ran some tests; and was somewhat surprised at the outcome. The module I'm currently working on now contains this documentation:

    Appending Bindings

    When appending bindings, using the ?+??script? format, the plus (+) isn't a separate argument. Any of the following syntax variations are valid (subname works for normal named subroutines as well as lexical subroutines).

    See Update below.

    '+' . sub { ... } '+' . \&subname '+' . [\&subname] '+' . [\&subname, @args] ['+' . \&subname] ['+' . \&subname, @args] '+' . $coderef '+' . [$coderef] '+' . [$coderef, @args] ['+' . $coderef] ['+' . $coderef, @args]

    The surprising part was all the different ways of concatenating a string ('+') with an anonymous coderef, a named coderef, and an anonymous arrayref, without the code blowing up in my face.

    This may also be useful to those using related modules, like Tcl::Tk and Tcl::pTk; although, I could be completely wrong on that (I have little knowledge of these beyond knowing of their existence).

    Update: Despite successfully running two dozen or so tests on all those syntax formats, none of them appear to be actually functional. My apologies to anyone who's been trying to get them to work.

    I've spent a bit of time looking into this. I can append one binding using either

    ... '+' . Tkx::i::interp->create_tcl_sub( CODEREF ) ...


    my $interp = Tkx::i::interp(); ... '+' . $interp->create_tcl_sub( CODEREF ) ...

    And that works for CODEREF as any of these three:

    sub { ... } \&subname $coderef

    However, whenever I attempt to append a second (or third) binding, none of the appended bindings work; although, the original binding works as expected. The only feedback I get looks like the following (there's no line numbers or other useful information):

    Error: invalid command name "::perl::CODE(0x7ffef5dd34d8)"

    So again, my apologies to anyone who rushed off to try what I originally posted. I will spend some more time on this: I'll let you know if that proves fruitful.

    — Ken

Spaces vs Tabs
9 direct replies — Read more / Contribute
by Tanktalus
on Jun 16, 2017 at 10:14

    Do you use spaces or tabs in your code for indentation?

    Apparently, StackOverflow says you get paid better for spaces than tabs!

    (I'm not sure whether to take this seriously or not, or, if serious, how much salt to consume simultaneously...)

    Me, I just let my editor do my indenting for me, and it produces spaces, so I'm good. :)

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