It was a matter of changing POST to 'POST'=>

For future reference, note that the => operator is just like the comma, except it does automagic quoting of its left-hand-side. This is made exactly for the case you've shown, to eliminate the need for quoting. In other words, your line could be written as:

$request = new HTTP::Request(POST => "...", 
                             [
                                 __EVENTTARGET   => "rdlZone_0",
                                 __EVENTARGUMENT => "",
                                 __VIEWSTATE     => "dDw0NzUyO",
                                 rdlZone         => "1",
                             ] );
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