Undefines the value of EXPR, which must be an lvalue. Use only on a
scalar value, an array (using @), a hash (using %), a subroutine
(using &), or a typeglob (using <*>). (Saying undef $hash{$key}
will probably not do what you expect on most predefined variables or
DBM list values, so don't do that; see delete.) Always returns the
undefined value. You can omit the EXPR, in which case nothing is
undefined, but you still get an undefined value that you could, for
instance, return from a subroutine, assign to a variable or pass as a
parameter. Examples:
undef $foo;
undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
undef @ary;
undef %hash;
undef &mysub;
undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
select undef, undef, undef, 0.25;
($a, $b, undef, $c) = &foo; # Ignore third value returned
Note that this is a unary operator, not a list operator.
Take our 
