if (defined $hash{$key}) { }

if (exists $hash{$key}) { }

my %hash = (a => 1, b => 0, c => undef, e=>"");

# key  value  defined  exists
a          1        1       1
b          0        1       1
c      undef        0       1
d      undef        0       0
e      undef        1       1



https://www.oreilly.com/library/view/perl-cookbook/1565922433/ch05s04.html

You want to remove an entry from a hash so that it doesn¡¯t show up with `keys`, `values`, or `each`. If you were using a hash to associate salaries with employees, and an employee resigned, you¡¯d want to remove their entry from the hash.

Use the `delete` function:

```
# remove $KEY and its value from %HASH
delete($HASH{$KEY});
```

Sometimes people mistakenly try to use `undef` to remove an entry from a hash. `undef` `$hash{$key}` and `$hash{$key}` `=` `undef` both make `%hash` have an entry with key `$key` and value `undef`.

The `delete` function is the only way to remove a specific entry from a hash. Once you¡¯ve deleted a key, it no longer shows up in a `keys` list or an `each` iteration, and `exists` will return false for that key.

This demonstrates the difference between `undef` and `delete`:

```
# %food_color as per Introduction
sub print_foods {
    my @foods = keys %food_color;
    my $food;

    print "Keys: @foods\n";
    print "Values: ";

    foreach $food (@foods) {
        my $color = $food_color{$food};

        if (defined $color) {
            print "$color ";
        } else {
            print "(undef) ";
        }
    }
    print "\n";
}

print "Initially:\n";
print_foods();

print "\nWith Banana undef\n";
undef $food_color{"Banana"};
print_foods();

print "\nWith Banana deleted\n";
delete $food_color{"Banana"};
print_foods();

Initially:

Keys: Banana Apple Carrot Lemon

Values: yellow red orange yellow

With Banana undef

Keys: Banana Apple Carrot Lemon

Values: (undef) red orange yellow

With Banana deleted

Keys: Apple Carrot Lemon

Values: red orange yellow ...
```

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