See Re: why can't compare a variable with undef directly? and perldata#Scalar values and page 23 of free book Modern Perl a loose description of how experienced and effective Perl 5 programmers work....You can learn this too.

The comparison works, only undef is undef, "" is never undef, "applesauce" is never undef and 42 is never undef and [] is never undef....