I do. To my mind, undef is not a value, it is the absence of a value. eq tests whether two values are equal, so applying it to the absence of a value (undef) seems rather odd.

Or, to look at it another way, what you're really trying to do is to determine whether $var is defined or not, so defined $var seems like the obvious way of doing so. $var eq undef looks to me like you're checking to see whether $var has a certain value rather than checking to see if it's defined.