#!/usr/bin/perl -l

use strict;

my $x;
print +($x eq '') ? 'Yes' : 'No';

##</code><code>##

#!/usr/bin/perl -l

use strict;

my $x;
print +(defined $x && $x eq '') ? 'Yes' : 'No';

##</code><code>##

if (
  (not defined $new and not defined $old)
  or 
  (defined $new and defined $old and $old eq $new)
) { ... }