@
''), 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 the delete manpage.) 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.