What are best practices for working with defined vs exists?

When working with Perl, understanding the difference between the `defined` function and the `exists` function is crucial for managing data structures properly, especially when dealing with hashes.

Best Practices:

• Use `exists` to check if a key is present in a hash, regardless of its value. This is useful when you want to determine if the key has been set or not.
• Use `defined` to check if a variable (including hash values) has been assigned a value and is not `undef`. This is important when you want to know if the variable holds a meaningful value.
• Avoid confusion by clearly commenting your code to specify when you are checking for key existence versus value definition.
• In performance-critical code, prefer `exists` for checking hash keys and use `defined` for checking variable values only when necessary.

Below is an example illustrating the difference between `defined` and `exists`: