How about trying explicit returns?

sub testUnless {
     my $v = 'Navidson';
     unless ( $v ) {
          return undef;
     }
     else {
          return $v;
     }
}
sub testIfNot {
     my $v = 'Holloway';
     if (!$v) {
          return undef;
     }
     else {
          return $v;
     }
}
[download]

You have to understand that Perl will return the last evalution from a sub if you are not explicit. So start being explicit!