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  1. or download this
      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.
    
  2. or download this
    #!/usr/bin/perl
    use warnings;
    ...
    my $x = '';
    print defined ($x), "\n";
    # prints a 1