You are viewing the version of this documentation from Perl 5.33.2. This is a development release of Perl.
undef EXPR
undef

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 $they_blew_it;
select undef, undef, undef, 0.25;
my ($x, $y, undef, $z) = foo();    # Ignore third value returned

Note that this is a unary operator, not a list operator.