It's not the fault of Alien::LibXML. It installs libxml and, if asked, supplies the location. The problem is that nobody is asking. (Alien::LibXML can't simply install libXML in, for example, /usr/lib, because it doesn't have the authority to do so. Also, different distros put libraries in different places and/or might have an older version the distro needs.)

Even when your module says it depends on Alien::LibXML and XML::LibXML, the cpan installer doesn't automatically tell each module it's installing about any other modules. If a module needs information about other modules, it has to get that information itself.

In the case of XML::LibXML, if it's Build.pl (or whatever it uses) can't find libxml, it could then try to require Alien::LibXML. If that succeeds, it could then ask Alien::LibXML where libXML is.

Update: However, I don't think there's a way to make a conditional dependency. If libxml were already installed, Alien::LibXML could skip installing it (unless the user specified otherwise), Alien::LibXML itself would still get installed if listed as a dependency by another module.