Each module is it's own Namespace.

The "cat" sub is declared in the "my_mod" namespace, and it is NOT in your "dog" script.

The reference needs to be explicit, unless you Export/import the names (a more advanced topic than you probably care for at the moment).

Try calling like this from 'dog":

  my_mod::cat();
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As you get more familiar with objects, you will probably want to create a "new" sub inside my_mod, and then create an instance of the class inside dog by doing:

  my $kitten = new my_mod;

  $kitten->cat(); # No need to qualify - $kitten already has access to
+ the my_mod namespace.
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