As covered in the require documentation, when require gets a string (instead of a bare statement), it doesn't automatically translate the '::' for you. You'll need to change your code to one of the following:

# Change the sprintf . . . 
$pkg = sprintf("%s/%s", $dir, $mod);

# . . . OR change the require to be inside an eval
eval "require $pkg";
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In the second form, the expanded variable looks like a bare statement to require, so the '::' translation works.