perldoc CGI
# skip some stuff
$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.