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.