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# Takes a hashref, a key name, and an optional default.
# Removes that key from the hash, and returns the value or
...
    confess("Missing required argument '$arg_name'");
  }
}

# Takes a hashref.  Verifies that it is empty
sub assert_args_done {
...
    confess("Unexpected arguments '@left' left over");
  }
}

sub some_func {
  my $args = { @_ };
...
  assert_args_done($args);
  # Rest of the code here.
}