require ${modulename};
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What's wrong with require $modulename; ?

#build the hash in an autoloaded module using a reference:
 &{${modulename}.'::$modulename'}($param1,\%myhash,\%hash2);
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Is that *REALLY* what's in your code? Are you *SURE* you're not getting errors doing that? Are you sure there is a subroutine literally named "$modulename" (note the $ at the start of the name) in the package named by $modulename ? Are you sure you understand what that code does? In any case, we'd probably need to see the code of that subroutine to help you out.

#in $usermodule, keys to the hash exist:
  @mykeys = keys %$hashref;  # is non-empty array
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That doesn't prove the keys you're looking for are there. Check that with exists.