That's a choice quote. Put another way, the OP is suffering from some confusion that if (defined $x) is intended to be the same as if ($x). In fact, they serve different purposes.

defined tests whether a variable has any value -- a false value is still a value.

Short demonstration code:

my $x;
demonstrate();

$x = 1;
demonstrate();

$x = 0;
demonstrate();

sub demonstrate {
  if (defined $x) { print "\$x has value $x\n"; }
  else { print "\$x is undefined\n"; }

  if ($x) { print "\$x evaluates as TRUE\n"; }
  else { print "\$x evaluates as FALSE\n"; }

  print "\n";
}
[download]

This produces the following output:

$x is undefined
$x evaluates as FALSE

$x has value 1
$x evaluates as TRUE

$x has value 0
$x evaluates as FALSE
[download]