$a = undef;
print $a ? 'true' : 'false';  # prints 'false'
print $a;                     # prints '' and warns
$a = 0;
print $a ? 'true' : 'false';  # prints 'false'
print $a;                     # prints '0'
$a = 'abc';
print $a ? 'true' : 'false';  # prints 'true'
print $a;                     # prints 'abc'

# whereas

$a = undef;
print !!$a ? 'true' : 'false';  # prints 'false'
print !!$a;                     # prints '' without warning
$a = 0;
print !!$a ? 'true' : 'false';  # prints 'false'
print !!$a;                     # prints ''
$a = 'abc';
print !!$a ? 'true' : 'false';  # prints 'true'
print !!$a;                     # prints 1