my @numbers = (1, 2, 2, 3, 3);
my @strings = qw(hello green grass rainbow pretty);
my @check   = (1, 2, 3, 4);

my @new_strings;

foreach (@numbers) {
    die "string $_ does not exists"
      unless defined $strings[$_-1];

    $new_strings[$_-1] .=  $strings[$_-1];
}

foreach (@check) {
    $new_strings[$_-1] = 'xxxxxx'
      unless defined $new_strings[$_-1];
}

print $_, "\n" for grep defined, @new_strings;