A scalar that contains no valid string, numeric, or reference
value is known as the undefined value, or
undef for short.
Many operations return the undefined value under exceptional
conditions, such as end of file, uninitialized variable, system error,
and such. This function allows you to distinguish between an
undefined null string and a defined null string when you're using operators
that might return a real null string.
You can use
defined to see if a subroutine exists, that is, if
the subroutine definition has been successfully parsed. However,
defined on an array or a hash is not
guaranteed to produce intuitive results, and should be avoided.