It's the code. If there are no values for a given parameter, then CGI->param() will return an empty list in list context. This is more or less necessary, because CGI has no way of knowing which parameters are expected to have a single value and which a list of values.

Unfortunately, this is a frequent gotcha in exactly the situation you show. (It's gotten me at least once!) The hash would also be messed up if the parameter returned more than one value. The solution is to call param() in scalar context, e.g.:

my %hash = (
            'key1' => 'value1',
            'key2' => scalar $r->param('doesnt_exist'),
            'key3' => 'value3',
);
[download]

or to create an anonymous array, if appropriate:

my %hash = (
            'key1' => 'value1',
            'key2' => [ $r->param('multi_valued') ],
            'key3' => 'value3',
);
[download]

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