my $h = {
            hello => [ 1, 2, 3, 4 ],
            world => [ 5, 6, 7, 8 ],
};

my $str = 'Hello World, Just another hello hack!';

for my $word( split /\W+/, $str ) {
    my $key = lc($word);
    my $mapping = exists $h->{$key} ? $h->{$key} : [ '#???#' ];
    print "$word\t@$mapping\n";
}

__DATA__
Hello	1 2 3 4
World	5 6 7 8
Just	#???#
another	#???#
hello	1 2 3 4
hack	#???#