Defined() in Perl returns true if the provided variable ‘VAR’ has a value other than the undef value, or it checks the value of $_ if VAR is not specified. This can be used with many functions to detect for the failure of operation since they return undef if there was a problem.
If VAR is a function or reference of a function, then it returns true if the function has been defined else it will return false if the function doesn’t exist. If a hash element is specified, it returns true if the corresponding value has been defined, but it doesn’t check for the existence of the key in the hash
VAR which is to be checked
Returns 0 if VAR is undef and 1 if VAR contains a value
X is defined Y is not defined
Function Exists Y is not defined