- When you build your Perl from source, configure it, and build it such that features you do not use (threads, for example) are not built.
- Having built and installed your Perl, locate the modules you use by searching the code for use Xxxx; statements, and their dependencies, which are all documented, or available by looking in their source. This is not an easy task.
- Delete Modules form your installation that you don't use. Be careful, keep backups.
The better answer is don't bother. Ship an entire standard Perl. The few MB in savings you could get may be outweighed by the task you must undertake to create a minimal Perl.