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*). Sayingundef $hash{$key} will probably not do what you expect on most predefined variables or DBM list values, so don't do that; seedelete. 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 returnedNote that this is a unary operator, not a list operator.
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