$ cat script.pl
use warnings;
use strict;

my ($host, %num);

while ( <> ) {
        next if /^\s*$/;
        chomp;
        do { $host = $_; next } if /^[^:]/;
        push @{ $num{ substr( $_, 1 ) } }, $host;
}                                                                                                                                                                                                                                            
                                                                                                                                                                                                                                             
for ( sort { $a <=> $b } keys %num  ) {                                                                                                                                                                                                      
        printf "%d: %s\n", $_, join ",", @{ $num{ $_ } };                                                                                                                                                                                    
}
$ perl script.pl infile                                                                                                                                                                                                    
1: host1,host2                                                                                                                                                                                                                               
2: host1,host2,host 3                                                                                                                                                                                                                        
3: host2,host 3                                                                                                                                                                                                                              
4: host2,host 3