I think that the only thing which you can realistically do is to document the requirement.   Put the prerequisite Perl package into the dependency list for yours, but tell the Gentle Reader that it is important that the underlying .so or .dll files be present.   cpan will recognize the prerequisite and attempt to install XML::LibXML if it isn’t there yet, but most-unfortunately that install might not be successful and there might be nothing that the (non-Administrative) user can actually do about it.   Some [Perl] installers react graciously when an underlying binary library isn’t there; others fall-down into a tangled heap of smelly ones and zeros.   Generally speaking, none of them can actually be expected to install the library.   So, I think, you need to voice the issue very clearly in your package documentation, including a discussion of what the Gentle Reader may expect (and that he might need to call the I.T. department).