If you're using CGI.pm (and you almost certainly should be :) ), there's a variable $CGI::POST_MAX which, if set, puts a ceiling on the amount of data that can be received in a POST.
This isn't very likely unless you've explicitly set it, so Jeffa's comments are most likely your best bet.
If you showed us some code, or at least told us how much data is too much, then we might be able to give more informed/relevant advice.
Hope this helps