Something like this:

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

my %data_needed;
my $key;

while (<DATA>) {
    chomp;
    if (/^(\d.+?)(\d{8,}.+?)?$/) {
        $key = $1;
        push @{ $data_needed{$key} }, $2;
    }
    else {
        s/^\s+//;
        push @{ $data_needed{$key} }, $_;
    }
}

{
    no warnings 'uninitialized';

    for ( keys %data_needed ) {
        my $space = " " x length($_);
        my $value = join "\n$space", @{ $data_needed{$_} };
        printf "%s%s\n\n", $_, $value
          if $value =~ /---/ or $value eq '';
    }
}

__DATA__
7 hostname12 Generic-legacy 10000000AB210ACF6 ---
                             10000000AB210ACF4 2:5:4
                             10000000AB210ACF4 2:3:4
                             10000000AB210ACF6 3:5:4
9 hostname13 Generic        10000000AB2A3006A 3:5:2
                             10000000AB2A30068 2:5:2
20 hostname14 Generic-legacy 10000000AB2A3000C ---
                             10000000AB2A3000E 3:3:1
21 HOSTNAME   Generic
22 hsname12   Generic-legacy 10000000ABCDE004A 3:3:3
                             10000000ABCDE004A 3:5:2
                             10000000ABCDE0048 2:3:3
23 srvernam   Generic-legacy 5001438002A3004A 3:3:3
                             5001438002A3004A ---
                             5001438002A30048 2:3:3
                             5001438002A30048 2:5:2
                             5001438002A30048 2:5:2
[download]

Output:

23 srvernam   Generic-legacy 5001438002A3004A 3:3:3
                             5001438002A3004A ---
                             5001438002A30048 2:3:3
                             5001438002A30048 2:5:2
                             5001438002A30048 2:5:2

21 HOSTNAME   Generic

7 hostname12 Generic-legacy 10000000AB210ACF6 ---
                            10000000AB210ACF4 2:5:4
                            10000000AB210ACF4 2:3:4
                            10000000AB210ACF6 3:5:4

20 hostname14 Generic-legacy 10000000AB2A3000C ---
                             10000000AB2A3000E 3:3:1
[download]

Note:
Please, note that the order of your output might be different to what I have here. Of course, the desired display order is left to the OP.
Hope this helps