libcgi | A C library to build CGI applications

Perl is an essential part of many Linux distributions, and has to come pre-installed. The system perl that is used by the operating system is usually installed as /usr/bin/perl. Modules for it are managed through the package manager (e.g. apt) and not via cpan/cpanm. If you were to install modules for the system perl yourself, this might conflict with modules expected by the operating system. Worse, installing the wrong module version could break parts of the OS. Similarly, replacing the system perl is a bad idea. That's why those Docker images install the different perl alongside.

For your apps, you should avoid the system perl. If you want to install extra modules for use with the system perl, consider using local::lib. In some cases you might install dependencies such as C libraries or external tools via apt, but you wouldn't use apt-provided Perl modules.

Unless you are targeting a specific operating system, do not hardcode the #!/usr/bin/perl shebang. Instead, prefer #!/usr/bin/env perl so that the script will use the perl that is first in the PATH. Alternatively, use wrapper scripts to explicitly invoke the correct perl installation. For example:

I got it to work by deleting all the MySQL files, which weren't deleted by apt-get purge mysql*: