Virtualenv is a great tool in Python that allows the user to separate the 3rd-part package installations of various projects.

When we have multiple projects on the same system using the same modules but requiring different versions we have a problem.

We need to be able to install two different version of the same module. Perl 5 does not provide a way to do this.

The solution with CPAN Minus

cd project_root/

export PERL5LIB=venv/lib/perl5
export PATH=venv/bin/:$PATH
cpanm -l venv/ --installdeps .

CPAN Minus is one of the clients of CPAN that makes module installation easy. We just type in 

cpanm Module::Name

and it will install the module.

If we have a Makefile.PL that lists all the dependencies of our project we can then use 

cpanm --installdeps .

to install all the dependencies of our project.

Using the -l flag we can tell cpanm where to install the modules.

If we would like to use the venv (any arbitrary directory name would work) directory inside the root of our project we can pass -l venv to cpanm and it will install the modules in that directory.

We also want to make sure the modules that were installed can be found. This is important any time a module is to be used, including when it is used during the installation of another module depending on it. So we assign the appropriate value relative to the venv directory: 

export PERL5LIB=venv/lib/perl5

before we try to install modules.

Finally, some modules will install executables in the venv/bin directory. We would want the command line to find them: 

export PATH=venv/bin/:$PATH