perldoc CGI
$CGI::POST_MAX
    If set to a non-negative integer, this variable puts a ceiling on
    the size of POSTings, in bytes. If CGI.pm detects a POST that is
    greater than the ceiling, it will immediately exit with an error
    message. This value will affect both ordinary POSTs and multipart
    POSTs, meaning that it limits the maximum size of file uploads as
    well. You should set this to a reasonably high value, such as 1
    megabyte.