file is not the name of any of the form fields listed in the HTML you posted. If you are trying to send a file through the FILE1 form field, for example, you need to change the field name passed to the Content portion of the call to $browser->post():

my $response = $browser->post(
  $url,
  Content_Type => "multipart/form-data",
  Content      => [FILE1 => ['data.txt']]
);
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