Quote the docs: "The optional fourth parameter (-label) is the user-readable label to be attached to the checkbox. If not provided, the checkbox name is used." Try

$query->checkbox(
   -name => 'copy',
   -label -> '',
   -value => 'off',
   -OnClick => ...
)
[download]

instead of

$query->checkbox(
   -name => 'copy',
   -value => 'off',
   -OnClick => ...
)
[download]