undef # evaluates to "" so it's false -- note, you may get a warni
+ng "use of uninitialized value if you are using -w
I belive you can not get such a warning, an undefined value
is always false without any warning when interpreted as a boolean.
From perlsyn, section Declarations (it's so well hidden):
A variable holds the undefined value ("undef")
until it has been assigned a defined value, which is anything other than "undef".
When used as a number, "undef" is treated as 0; when used as a string, it is
treated as the empty string, ""; and when used as a reference that isn't being
of an uninitialized value whenever you treat "undef" as a string or a number.
Well, usually. Boolean contexts, such as
my $a;
if ($a) {}
are exempt from warnings (because they care about truth rather than definedness).
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