=over =item defined EXPR X X X =item defined Returns a Boolean value telling whether EXPR has a value other than the undefined value L|/undef EXPR>. If EXPR is not present, L|perlvar/$_> is checked. Many operations return L|/undef EXPR> to indicate failure, end of file, system error, uninitialized variable, and other exceptional conditions. This function allows you to distinguish L|/undef EXPR> from other values. (A simple Boolean test will not distinguish among L|/undef EXPR>, zero, the empty string, and C<"0">, which are all equally false.) Note that since L|/undef EXPR> is a valid scalar, its presence doesn't I indicate an exceptional condition: L|/pop ARRAY> returns L|/undef EXPR> when its argument is an empty array, I when the element to return happens to be L|/undef EXPR>. You may also use C to check whether subroutine C has ever been defined. The return value is unaffected by any forward declarations of C. A subroutine that is not defined may still be callable: its package may have an C method that makes it spring into existence the first time that it is called; see L. Use of L|/defined EXPR> on aggregates (hashes and arrays) is deprecated. It used to report whether memory for that aggregate had ever been allocated. This behavior may disappear in future versions of Perl. You should instead use a simple test for size: if (@an_array) { print "has array elements\n" } if (%a_hash) { print "has hash members\n" } When used on a hash element, it tells you whether the value is defined, not whether the key exists in the hash. Use L|/exists EXPR> for the latter purpose. Examples: print if defined $switch{D}; print "$val\n" while defined($val = pop(@ary)); die "Can't readlink $sym: $!" unless defined($value = readlink $sym); sub foo { defined &$bar ? $bar->(@_) : die "No bar"; } $debugging = 0 unless defined $debugging; Note: Many folks tend to overuse L|/defined EXPR> and are then surprised to discover that the number C<0> and C<""> (the zero-length string) are, in fact, defined values. For example, if you say "ab" =~ /a(.*)b/; The pattern match succeeds and C<$1> is defined, although it matched "nothing". It didn't really fail to match anything. Rather, it matched something that happened to be zero characters long. This is all very above-board and honest. When a function returns an undefined value, it's an admission that it couldn't give you an honest answer. So you should use L|/defined EXPR> only when questioning the integrity of what you're trying to do. At other times, a simple comparison to C<0> or C<""> is what you want. See also L|/undef EXPR>, L|/exists EXPR>, L|/ref EXPR>. =back