Well, unless you really need to remind yourself that you're sorting lexically, drop the sort sub -- lexical is the default sort:

#!/usr/local/bin/perl -w
use strict;

my @data = (<DATA>);
print "Naked...\n\n";
for (sort @data) { print }
print "\nNow defined....\n";
for (sort {$a cmp $b} @data) { print }
exit;
__END__
1
3
 2
merlyn
4
10
birthday
12
 9
[download]

Of course that doesn't address the "elegance" issue of the two undef values, but you already have a good answer for that :)