..then the linking won't take place, simply because MakeMaker has removed that '-lxml2' link.

That's absolutely correct, and I wasn't questioning that  (I was just commenting on your focus on the .a file :)

As to the contradicting Makefile.PL messages (the have_library() routine obviously did find it!), I think it would be best if the OP simply looked at which commands are actually being run by the Makefile, in order to figure out whether the -lxml2 together with an appropriate -L/... is there on the command line, as it should.