#Error here: Useless use of hash element in void context
    $singleton->{$XMLfile_name}=>$_;
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something => another_thing means "something", another_thing (note the quotes, see Comma Operator for more info). Did you mean just "="?

You also seem to use $_ instead of elements in @_. Fuction arguments are in the @_ array (which is modified by shift, by the way), not in the single $_ scalar variable.