None of the solutions do what you asked--although they may do what you want. That's because a key can exist, yet its corresponding value is undef:

use strict;
use warnings;
use 5.010;


my %hash = (
    'one' => undef,
);

say "The key 'one' exists" if exists $hash{one};
say "The key 'two' exists" if exists $hash{two};

--output:--
The key 'one' exists
[download]

Note that nothing prints out for the non-existent key: 'two'. Then if you do this:

$hash{one} ||= 1;
say $hash{one};

--output:--
1
[download]

you will assign 1 to a key that exists--which does not meet your stated requirement.