Your problem is that PAR wraps up all the files into the single .par file; which is essentially a .zip file. From the PAR documentation:

This module lets you use special zip files, called Perl Archives, as libraries from which Perl modules can be loaded. ... A .par file is mostly a zip of the blib/ directory after the build process of a CPAN distribution

So, a simply search of your file system will not look inside any .par file dependancies. You would need to use some kind of .zip browser or unzip utility to look inside them.

As a crude test, you might try:

findstr /s /m perl510.dll c:\*.par
[download]

Note:That will not be fast. But it should give you a list of .par files that contain a reference to the errent dll.

So I doubt that any "other DLL" exists...

They must. Or you OS is lying to you.