$html .= $query->scrolling_list(
  -name    => 'my_parameter',
  -values  => [qw/option1 option2/],
  -default => $existing_value,
  -labels  => { option1 => 'Option 1',
                option2 => 'Option 2'},
  -multiple => 'true',
);

# or

$html .= $query->popup_menu(
  -name    => 'my_parameter',
  -values  => [qw/option1 option2/],
  -default => $existing_value,
  -labels  => { option1 => 'Option 1',
                option2 => 'Option 2'},
);