There are several ways. But here's the easiest:

use lib '/foo/bar';
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Adds the path '/foo/bar' to your @INC so it'll be searched. So, for your purposes, if you had installed Tie::CaseInsensitive (or Tie::RecursiveCI, japhy's code, which you should use because then you don't have to retie each level) in '/home/foo/bar/Tie/RecursiveCI.pm', you'd put

use lib '/home/foo/bar';
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at the top of your code.