I also know that the path is available in @INC (via use lib...). But I don't know how to get the correct one from here.

Using DBI.pm as an example of a file to find:

#!/usr/bin/env perl 
use strict;
use warnings;

my ($path) = grep { -f "$_/DBI.pm" } @INC;
print $path;
[download]

Not absolutely the most efficient, but it will work.