Frankly, I'm embarrased, because I'm BFI'ing it, instead of doing things properly.

But here goes. Against my better judgement.

#
#  WARNING WARNING WARNING WARNING
#
#  USE AT YOUR OWN RISK.
#
#  THIS IS A MASSIVE KLUDGE.
#
#  YOU HAVE BEEN WARNED.
#
my $in = <DATA>;

# ASSUME sentences end in a period and a space.
my @sentences = split '\. ', $in;

foreach( @sentences )
{
   # ASSUME these words are mostly useless
   # for our purposes...
   s/\b(with|a|of|the|in|just)\b//gi;
   
   # ASSUME phrases are comma-separated.
   my @phrases = split ',';
   
   my @subjects = ();
   my @descs    = ();
   
   foreach ( @phrases )
   {
      s/^\s*//; # trim leading spaces.
      s/\n//g;  # remove newline.
      
      # Well, do we have a subject, or a descriptor?
      
      # ASSUME subjects are capitalized (!!)
      push @subjects, $_ if /^[A-Z]/;
      
      # ASSUME descriptions are not.
      push @descs,    $_ unless /^[A-Z]/;
   }
   
   # Print 'em all out.
   foreach my $subj ( @subjects )
   {
      my @subsub = ($subj);
      # ASSUME 'and' separates multiple subjects (!!)
      @subsub = split ' and ', $subj if $subj =~ /\band\b/;
      
      foreach my $ss (@subsub)
      {
         print "$ss: $_\n" foreach @descs;
      }
   }
}

__DATA__
With a population of more than 10.2 million, Seoul, the capital of Sou
+th Korea, is the world's largest city in terms of population. Sao Pau
+lo(Brazil), the world's second-largest city, has a population of just
+ over ten million. Three other cities, Bombay(India), Jakarta(Indones
+ia) and Karachi(Pakistan), have grown to more than nine million peopl
+e.
[download]

The output:

Seoul: population  more than 10.2 million
Seoul: capital  South Korea
Seoul: is  world's largest city  terms  population
Sao Paulo(Brazil): world's second-largest city
Sao Paulo(Brazil): has  population   over ten million
Three other cities: have grown to more than nine million people.
Bombay(India): have grown to more than nine million people.
Jakarta(Indonesia): have grown to more than nine million people.
Karachi(Pakistan): have grown to more than nine million people.
[download]