Difference Between 'defined' and 'exists' in Perl

# Initialize a hash key 'foo' with a standard string value

$hash{'foo'} = 'bar';

# 'defined' checks if the value is something other than undef. 

# Since 'bar' is a valid string, it evaluates to true.

print defined $hash{'foo'};      # prints 1

# 'exists' checks if the key 'foo' is present in the hash, regardless of its value.

# Since we created the key above, it evaluates to true.

print exists $hash{'foo'};       # prints 1

# The subtle difference appears when a key is explicitly set to the 'undef' value:

$hash{'baz'} = undef;

# 'defined' evaluates the VALUE. Since the value is undef, this returns false.

print defined $hash{'baz'};      # doesn't print 1 (returns false/empty string)

# 'exists' evaluates the KEY. The key 'baz' has been created in the hash, 

# so it exists, even though its assigned value is undefined.

print exists $hash{'baz'};       # prints 1