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.
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An interesting thing to notice, is that defined returns nothing if something is undef. But it prints a 1 if defined. Sort of an asymetric boolean. One might expect 0 to be printed, but that return value would be defined, and would screw up the defined function used in more complex logic.

#!/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
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