---
name: undef
status: documented
runtime: pp
source: src/runtime/pp/array_ops.rs
---
```{index} single: undef; Perl built-in (pp runtime)
```

# undef

## Synopsis

```perl
my $x = undef;
undef $x;                    # set $x to undef
undef @array;                # clear the array
undef %hash;                 # clear the hash
my (undef, @rest) = @list;   # skip first element in list assignment
```

## Description

Produces the undefined value, or undefines an existing variable.

Operates in three modes determined by op flags:

- **MOD (lvalue placeholder):** Used in list assignment patterns like
  `my (undef, $x) = @list`. Adds a sentinel to `lvalue_targets` so
  `pp_aassign` skips that position.
- **STACKED (function call `undef $x`):** Pops the target from the
  stack and undefines it. For arrays and hashes this clears the
  container; for hash/array element targets it sets the element to
  `undef`; for scalars it writes `undef` through the alias.
- **Default:** If `targ` is set, sets the corresponding pad slot to
  `undef`. Always pushes `undef` onto the stack as the return value.

## See also

defined