It's not a hash (reference), it's an array reference. Note the square brackets (not curly braces). 
my $post_elements = [
  ...
];

my $response = $browser->post($udr, $post_elements, @netscape_headers)
+;

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The reason it's not a hash ref is to easily allow (without the need for references) multiple values for a single key.
Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart

In reply to Re: Making a Data Structure - Example by japhy
in thread Making a Data Structure - Example by awohld

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