Just use a hash to map the word to the array. The array will be stored as an array ref and the keys are case sensitive. See perlman:perlreftut.

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    #???#
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