Depending upon the directions which you take with development, this could be a bigger project than anticipated. If I were tackling this project my first task would be to develop a schema of installation steps, both prior to and following the copying of files, allowing for the searching for dependencies and execution of external script or configuration components - This schema to my mind would need to be modular in fashion so that installation behaviour can be defined by a series of definition or configuration files such that your installer script would offer you the greatest flexibility without having to go back and constantly modify/update it.

On to your specific questions however ...

1. What's the fastest/most efficient (without wasting memory) way to check if a module is installed?

   To my mind, the quickest would be to actually try and use the module - For example:

   eval {
       require $module;
   }
   if ($@) {
       #   handling for when module fails to load
   }
   [download]

   This approach to module and library handling is discussed in the perlfunc:require man page.

    

2. What happens when a program tries to use 'CPAN' but noone has configured it?

   The init method from the module CPAN::FirstTime is invoked which attempts to configure the CPAN module by prompting the user for information and writing a CPAN::Config configuration file.

    

3. Is there any way to determine the path to sendmail? Or is there a common default path?

   While the majority of sendmail installations use the target location of /usr/sbin/sendmail, this is not absolute and should not be relied upon. A better way would be to search for the sendmail binary within your script, making use of the File::Find module. However, for better cross-platform compatibility, it may be better still to prompt the user for the local network mail server and communicate directly via SMTP - The advantage which this has is that your script installation is not dependent upon a local sendmail installation for execution and offers greater flexibility to administrators who may wish to impose local mail security policies.

    

Good luck! :-)