Among the other properties of &CGI::checkbox_group is {linebreak=>'true'}. The list of @predefined should be an arrayref:

print $q->checkbox_group (
    -name=>'predefined',
    -values=>\@predefined,
    -linebreak=>'true'
    -default=>[ @predefined[@slicer] ],
    -labels=>\%predefined_labelhash,
    );
[download]

Those are all documented in 'perldoc CGI'.

Update: Corrected the omitted '-' in the arguments. Here is a working test:

#!/usr/bin/perl -w
use strict;

use CGI;

my $q = CGI->new();
my @predef = ( 'abba', 'dabba', 'doo');
my @slicer = ( 1 );
my %labels = ( abba => 'pop', dabba => 'amateur', doo => "don't go the
+re" );

print $q->checkbox_group ( -name => 'predefined',
   -values => \@predef,
   -default => [@predef[@slicer]],
   -linebreak => 'true',
   -labels => \%labels,);

=pod comment
$ perl ~/tcgi.pl
<input type="checkbox" name="predefined" value="abba" />pop<br /><inpu
+t type="checkbox" name="predefined" value="dabba" checked="checked" /
+>amateur<br /><input type="checkbox" name="predefined" value="doo" />
+don&#39;t go there<br />
$
=cut
[download]

xtype, maybe your @predefined array doesn't contain what you think it does.

After Compline,
Zaxo