if (exists($h{x} && defined($h{x}) && $h{x})

##</code><code>##

if ($h{x})

##</code><code>##

if (defined($s) && length($s))

##</code><code>##

if (length($s))

##</code><code>##

my $length = length($undef);  # Used to warn here.
print $length;                # Still warn here.

my $length = length($undef);  # Used to warn here.
for (1..$length)              # Still warn here.

my $length = length($undef);  # Used to warn here.
pack 'N', $length             # Still warn here.

...