You can use the built-in defined function to explicitly check if a variable is defined, so e.g.
if( defined( $rttjitter ) and $rttjitter ) { ...
Will check that it's defined and has a value that is not 0 or an empty string.
It seems likely though you should take another look at how you're reading in your CSV (you are using a module like Text::CSV, right?), and/or how you're doing the DB insertion, as generally with this sort of thing the undef value will cleanly insert as a NULL into the DB and should not cause any problems with your code, unless you're doing something strange.