There are several alternatives. You could set the directory name to environment variable PERL5LIB.

Or you could:

use lib '\etc\usr\lib';
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If I understood your tree correctly. It is best to use the full path name, in case scripts are moved.
If you don't like hard-coding a path, and you don't like PERL5LIB then you can use your own environment variable:

use lib $ENV{MY_PERL_LIB};
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Alternatively you can manipulate @INC directly in a BEGIN block, but you probably want to avoid that unless you have a good reason.