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  * passing scalars as themselves

...

  * passing a "reference to a hash" (which is itself a scalar)
      instead of the hash itself

  sub do_something($my_scalar, $my_arrayref, $my_hashref) {
    my @my_array = @$my_arrayref;
    my %my_hash  = %$my_hashref;
    ### more code here ###
  }