Here's how I'd approach it:

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

    my %concat;

    for (0 .. $#numbers) {
      $concat{$numbers[$_]} .= $strings[$_];
    }

    for (@check) {
      print $concat{$_} || "xxxxxxxx", "\n";
    }
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