When should you prefer defined vs exists, and when should you avoid it?

In Perl, the choice between defined and exists plays a crucial role in handling variables and their states, especially when dealing with hashes.

Use defined when:

You need to check if a scalar variable has been assigned a value and is not undef.
You're working with arrays or scalars where undef could be a valid state you want to differentiate.

Use exists when:

You are checking for the presence of a key in a hash, regardless of the value it holds.
You're working with hashes that may contain keys with undef values, and you still want to know if the key exists.

Avoid using defined and exists interchangeably:

Using defined on a hash key will return false if the key exists but has been set to undef.
exists will return true for a key that has been defined, even if its value is undef.

Here’s a practical example:


        # Perl Example
        my %hash = (a => 1, b => undef);
        
        if (exists $hash{"b"}) {
            print "'b' exists in the hash\n";  # This will print
        }

        if (defined $hash{"b"}) {
            print "'b' is defined\n";  # This will NOT print
        }

defined exists Perl hash check variable hash key presence