Why can't I simply copy installed Perl modules to other machines?

Question

Being very new to Perl but not to dynamic languages, I'm a bit surprised at how not straight forward the manage of modules is.

Sure, cpan X does theoretically work, but I'm working on the same project from three different machines and OSs (at work, at home, testing in an external environment).

• At work (Windows 7) I have problem using cpan because of our firewall that makes ftp unusable
• At home (Mac OS X) it does work
• In the external environment (Linux CentOs) it worked after hours because I don't have root access and I had to configure cpan to operate as a non-root user
• I've tried on another server where I have an access. If the previous external environment is a VPS and so I have a shell access, this other one is a cheap shared hosting where I have no way to install new modules other than the ones pre-installed

At the moment I still can't install Template under Windows. I've seen that as an alternative I could compile it and I've also tried ActiveState's PPM but the module is not existent there.

Now, my perplexity is about Perl being a dynamic language. I've had all these kind of problems while working, for example, with C where I had to compile all the libraries for all the platform, but I thought that with Perl the approach would have been very similar to Python's or PHP's where in 90% of the cases copying the module in a directory and importing it simply works.

So, my question: if Perl's modules are written in Perl, why the copy/paste approach will not work? If some (or some part) of the modules have to be compiled, how to see in CPAN if a module is Perl-only or it relies upon compiled libraries? Isn't there a way to download the module (tar, zip...) and use cpan to deploy it? This would solve my problem under Windows.

Answer 1

Now, Perl is a dynamic language, but that doesn't imply that everything that people write is portable across platforms. That's not the fault of the language. It's not even the fault of the programmer. Some things, like Win32::OLE shouldn't work on Unix. :)

Other dynamic languages will have some of the same problems. If you have to compile C code, you won't be able to merely copy files to another machine. Some distributions configure the code slightly differently depending on your operating system, etc.

Even if you could copy files, you have to ensure that you copy all of the files that you need. Do you know everything that you need for a particular module? Remember, many of them have dependencies.

Most of the problems you're having aren't anything to do with the language. You're having trouble with the tools. If you want a zero conf CPAN tool that makes all the decisions for you, try cpanminus. It's mostly the same thing that you'd get out of cpan (although different code), but it makes all of the decisions for you. It doesn't run any of the distribution tests, and it installs into your user directory. When you need something that gives you control, come back to cpan.

Answer 2

In the external environment (Linux CentOs) it worked after hours because I don't have root access and I had to configure cpan to operate as a non-root user

This is one of those times when it helps to know The Trick. In this case local::lib, which lets you configure a non-root install area and all the ENV variables in about three minutes.

if perl's modules are written in perl, why the copy/past approach will not work?

Some are written in Pure Perl, but many are written partially in C (using Perl's XS API) for efficiency.

Sometimes you end up with situations like JSON::XS, JSON::PP and JSON::Any to autoselect the best one that is installed.

Isn't there a way to download the module (tar, zip...) and use cpan to deploy it?

The cpan program is all about getting things from the Internet. You can download the package (there will be a link along the lines of "Download: CGI.pm-3.49.tar.gz" on the right hand side of the CPAN page), untar it, then

perl Makefile.PL
make
make install

You would probably be better off configuring your cpan installation to use only HTTP sources (in the urllist config option). Possibly going to far as to create a mini CPAN mirror inside your network.