Congratulations, you've found another bug in CGI.pm's html generation subroutines, hooray, you should report it :)

#!/usr/bin/perl --

use strict;
use warnings;

use CGI::Pretty;

my %labels = (
    'eenie'  => 'your first choice',
    'meenie' => 'your second choice',
    'minie'  => 'your third choice'
);

print "\n------ broken (the example in the docs), unsurprising, its un
+tested ----------\n";
print CGI::Pretty->checkbox_group(
    'group_name',
    [ 'eenie', 'meenie', 'minie', 'moe' ],
    [ 'eenie', 'moe' ],
    'true',
    \%labels,
    {
        'moe' => { 'class' => 'red' },
        'eenie' => { 'class' => 'yo' },
    }
);

print "\n----- works (also documented but untested) -----------\n";
print CGI::Pretty->checkbox_group(
    -name => 'group_name',
    -values => [ 'eenie', 'meenie', 'minie', 'moe' ],
    -default => [ 'eenie', 'moe' ],
    -linebreak => 'true',
    -labels => \%labels,
    -attributes => {
        'moe' => { 'class' => 'red' },
        'eenie' => { 'class' => 'yo' },
    }
);


__END__

------ broken (the example in the docs), unsurprising, its untested --
+--------
<label eenie="HASH(0xa7d7a4)" moe="HASH(0xa7d6b4)">
        <input type="checkbox" name="group_name" value="eenie" checked
+="checked" />your first choice
</label>
<br /><label eenie="HASH(0xa7d7a4)" moe="HASH(0xa7d6b4)">
        <input type="checkbox" name="group_name" value="meenie" />your
+ second choice
</label>
<br /><label eenie="HASH(0xa7d7a4)" moe="HASH(0xa7d6b4)">
        <input type="checkbox" name="group_name" value="minie" />your 
+third choice
</label>
<br /><label eenie="HASH(0xa7d7a4)" moe="HASH(0xa7d6b4)">
        <input type="checkbox" name="group_name" value="moe" checked="
+checked" />moe
</label>
<br />
----- works (also documented but untested) -----------
<label>
         <input type="checkbox" name="group_name" value="eenie" checke
+d="checked"  class="yo"/>your first choice
</label>
<br /><label>
         <input type="checkbox" name="group_name" value="meenie" />you
+r second choice
</label>
<br /><label>
         <input type="checkbox" name="group_name" value="minie" />your
+ third choice
</label>
<br /><label>
         <input type="checkbox" name="group_name" value="moe" checked=
+"checked"  class="red"/>moe
</label>
<br />
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