defined EXPR
       defined Returns a Boolean value telling whether EXPR has
 a value other than the undefined value "undef".  
If EXPR is not present, $_  is checked.

##</code><code>##

#!/usr/bin/perl
use warnings;
use strict;

print  defined (undef) ,"\n";
# prints nothing but a newline

my $y = undef;
print defined ($y),"\n";
# prints nothing but a newline

my $x = '';
print defined ($x), "\n";
# prints a 1