The most likely thing to happen is that there is more than one Perl on your machine. If so, then you installed JSON in one Perl instance and were in fact using another.

This can happen when the system Perl isn't the commonly used Perl. System packages install in the system Perl.

The module library directories(locations shown via the array @INC) are separate for separate Perls. When you implemented CPAN for the Perl you are used to using, it then installed a module for the Perl you use.

Now, I haven't proven this bit of speculation, but it seems pretty logical, given the nature of your issues.

David