my @words = qw(sem sema cat semok hgt cato
                 seme cate);
  my %similar = ();
  foreach my $word ( @words )
  {
      push( @{$similar{substr($word,0,3)}}, $word );
  }

  foreach my $prefix ( sort keys %similar )
  {
      print( join( ', ', @{$similar{$prefix}} ) );
      print( "\n" );
  }

##</code><code>##

cat, cato, cate
hgt
sem, sema, semok, seme