Actually,

print
  $q->checkbox_group
  (
    -name=>'predefined',
    -linebreak=>'true',
    -values=>\@predefined,
    -defaults=>param('predefined'),
  );
[download]

does not appear to work. And instead fills the value and label of one checkbox with the arrayref. It returns something like:

<input type="checkbox" name="predefined" value="ARRAY(0x833ced0)" />AR
+RAY(0x833ced0)<br />
[download]

Where as:

print
  $q->checkbox_group
  (
    -name=>'predefined',
    -linebreak=>'true',
    -values=>@predefined,
    -defaults=>param('predefined'),
  );
[download]

Works perfectly well. And acts exactly the same as changing $, did. (except xhtml-ish br tag).

<input type="checkbox" name="predefined" value="element content" />ele
+ment content<br />
[download]

The perldoc appears to say one thing and do another.
Even in its' own examples...
"...the second argument should be an array reference." and yet:

print $query->checkbox_group(-name=>'group_name',
                                       -values=>['eenie','meenie','min
+ie','moe'],
                                       -default=>['eenie','moe'],
                                       -linebreak=>'true',
                                       -labels=>\%labels);
[download]

The perldoc also gives this example:

@h = $query->checkbox_group(-name=>'group_name',-values=>\@values);
&use_in_creative_way(@h);
[download]

But this just fills @h with one element.

print $h[0];
---
<input type="checkbox" name="group_name" value="ARRAY(0x833d000)" />AR
+RAY(0x833d000)
[download]

Very odd indeed.