Are you sure it's an error, not just a warning?   __EVENTTARGET presumably is a hidden field, which, according to HTML::Form, starts out readonly:

$bool = $input->readonly
$input->readonly( $bool )
This method is used to get/set the value of the readonly attribute. You are allowed to modify the value of readonly inputs, but setting the value will generate some noise when warnings are enabled. Hidden fields always start out readonly.

In reply to Re^5: Help with javascript navigation by almut
in thread Help with javascript navigation by bauer1sc

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