Anonymous Monk is correct, you can’t access key3 or 'Hello' (see perlref).

However, for the record, key4 and 'World' can be accessed as follows:

#!/usr/bin/perl
use strict;
use warnings;

my $hash =
{
    key1 =>
    {
        key2 =>
        {
            [ key3 => 'Hello' ],
            { key4 => 'World' },
        }
    }
};

my ($value2) = values $hash->{key1}{key2};
my ($key4, $value4) = each %$value2;

print "\$key4 = '$key4', \$value4 = '$value4'\n";
[download]

Output:

$key4 = 'key4', $value4 = 'World'
[download]

HTH,