And I suppose that manually installed modules have another @INC path, but your settings may vary.

ExtUtils::Installed looks like a good place to start. From the documentation:

ExtUtils::Installed provides a standard way to find out what core and module files have been installed. It uses the information stored in .packlist files created during installation to provide this information. In addition it provides facilities to classify the installed files and to extract directory information from the .packlist files.

I got this one-liner from here

 perl MFile::Find=find -MFile::Spec::Functions -e 'find {wanted=>sub{p
+rint canonpath $_ if /\.pm\z/}, no_chdir =>1}, @INC;'
[download]

And one way to do this is to run this on your target, vanilla installed instance and compare the output of your existing, modified system for differences.

Prior to your upgrade, you might want to get, and install HTML::Perlinfo. Then create the following, and open it in your web browser (other possibilities exist for the CLI)

#!/usr/bin/perl -T

use HTML::Perlinfo;
use strict;

print "Content-type: text/html\n\n";

perlinfo();
[download]

This will give you a wealth of information, regarding your current Perl installation. Not the least of which, are all the Modules you currently have installed. Think of it, as Taking Inventory. :)

BEWARE do delete the script I provided above, after you've used it. Because you DO NOT want others to have access to this information. :)

Best wishes.

--Chris

Another option:

PerlBrew, and I would strongly suggest it. It is easy to install/set up, and if your LTS upgrade does not destroy your $HOME dir, you are in the same spot with Perl as you were before the upgrade with the exception of the ditro's Perl, which you probably won't use much any more anyway. If the LTS does destroy $HOME, just back up $HOME prior to upgrade, which would be a good idea anyway.

Hope you find that helpful....

The list of core perl modules changes; vendors add/remove their own modules, and sometimes installvendorlib is same as installsitelib ( http://p3rl.org/INSTALL#Installation-Directories )

The only safe move is to get a list of all installed modules and their versions, which is called an bundle ... links on autobundles and things like that see Re^3: Is there an easier way to find and install modules on Linux? (metacpan autocompletion)