It's likely to be #2, since:

$ret = foo($scalar);
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Is (generally) equivalent to:

$ret = foo($scalar, undef, undef);
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There's another option, though, and it all depends on how the author wrote his/her code:

$ret = foo($scalar, [], $hash_ref);
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In other words, it may be necessary to use an empty arrayref, though my gut would say using just undef would be more likely correct.