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    # Derive the template's filename from all characters in the
    # requested URI
...
        # Write some code to generate a list of users on the site and
        # set some template parameters
    }

if ($template->query(name => 'temperature' or $template->query(name =>
+ 'cloud_cover') {
   # The code to find out what the weather is like goes here
}

my %param= map($_ => 1) $template->query();

if (exists $param{'temperature'} or exists $param{'cloud_cover'}) {
    # The weather code goes here
}