Programming Perl, 4th Edition

undef

undef EXPR
undef

undef is the name by which we refer to the abstraction known as “the undefined value”. Conveniently, it also happens to be the name of a function that always returns the undefined value. We happily confuse the two.^[255]

Coincidentally, the undef function can also explicitly undefine an entity if you supply its name as an argument. The EXPR argument, if specified, must be an lvalue. Hence, you may only use this on a scalar value, an entire array or hash, a subroutine name (using the & prefix), or a typeglob. Any storage associated with the object will be recovered for reuse (though not returned to the system, for most operating systems). The undef function will probably not do what you expect on most special variables. Using it on a read-only variable like $1 raises an exception.

The undef function is a unary operator, not a list operator, so you can only undefine one thing at a time. Here are some uses of undef as a unary operator:

undef $foo;
undef $bar{"blurfl"};   # Different from delete $bar{"blurfl"};
undef @ary;
undef %hash;
undef &mysub;
undef *xyz;             # destroys $xyz, @xyz, %xyz, &xyz, etc.

Without an argument, undef is just used for its value:

select(undef, undef, undef, $naptime);

return (wantarray ? () : undef) if $they_blew_it;
return if $they_blew_it;  # same thing

You may use undef as a placeholder on the left side of a list assignment, in which case the corresponding ...