Using Warnings

It's always good to have warnings enabled, so that u can get to know that u are doing something wrong. It's always worth spending some time wondering why?
A few things that trigger warning:

1. Trying to read from or write to an unopened filehandle, socket or device.
2. Treating a string of non-numeric characters as if it were a number.
3. Printing or performing comparisons with undefined values.
4. Assigning an odd number of elements to a hash (collection of key-value pairs).