d6c27972_Perl_5_by_Example__Functions_Table_5_2_Array_Functions_Description
[Function] defined(VARIABLE) [Description] 
returns true if VARIABLE has a real value and false if the variable has not yet been assigned a value. This is not limited to arrays, any data type can be checked. Also see the exists function for information about associative array keys.
Perl 5 by Example: Functions
Description