If it's for debugging, another way to tell the difference between the empty string and an undef value is to use Data::Dumper or Data::Dump. The first one requires no installation, but one of the reasons I like the second one better is because it displays non-printable character explicitly with escape sequences by default (it's the Useqq option in Data::Dumper), so it helps tell the difference between undef, "", "\n" or even "\0". As far as I'm concerned, whenever it's for debugging I don't print it I dump it.

Edit: eg:

use Data::Dump qw( pp );
pp undef;
pp "";
pp "\n";
pp "\0";
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NB: of course those two modules are also useful for displaying complex data structures.