That's why I indicated that using a directory relative would make it easier to diagnose/implement.

If both of your custom JSON modules declare

package JSON;
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You should, as toolic initially asserted, be able to use

use lib '/abc/xyz';
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or

use lib ('/abc/xyz');
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then

use JSON;
use Test::JSON
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because as I noted earlier; the double-colon is a path/directory separator. Meaning; Test::JSON == Test/JSON.

You might also try

use lib '/abc/xyz/';
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as that negates the need to preface a slash before your custom modules.

In any case; you must ensure that the path to your custom JSON modules preceeds (is before) your system Perl module path.
That's why you see reference to conflicts within the system's perl, in the latest error you quoted.

--Chris