group_create(\%hash);
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and you're doing:

$userdb->group_create ( $group->{groupname} )
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$group->{groupname} isn't a hashref, in fact AFAICT it is the value 'wheel'. Try this:

$userdb->group_create($group);
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or

$userdb->group_create({groupname=>$group->{groupname}});
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if you don't want to pass the rest of the contents of %$group in.