What you've told us leaves me less than confident that I understand what the actual problem is. See I know what I mean. Why don't you?. But here are a few WAGs/WAIs:

• Make the parameter required (since it appears the log tells you that some visitor missed a checkbox, input field or some such) in the html.
• Consider using CGI::Validate or Params::Validate
• Move your $c_enabling=''; above (eg, before) the code you've shown (and move the my to the same line and put a test of the parameter's existance ahead of line 1 along code to tell the user than the param must be supplied, at which point $c_enabling won't be uninitialized and the visitor will be cautioned against omitting a field.

Of course, if I've mis-diagnosed your problem, the above may be mere garbage....