I use the code below to shorten an unusually long topic (hackish, perhaps) so that it doesn't mess the page's layout.

my $str = trim_length("thisisaverylongsentencewithoutspacesinbetween",
+ 30);
print "$str\n";

sub trim_length {
   my ($str, $desired_len) = @_;
   my $tmp_len = $desired_len;
   # Shorten $str if it's is one continuous string
   # and its length is greater than the desired length
   if ($str !~ /\s/ && length($str) > $desired_len) {
      $str = substr($str, 0, $desired_len);
      $str .= ' ...';
   }
   # Otherwise, if the length of $str is greater than the
   # desired length, iterate over each character of in $str
   # to find an appropriate spot that's a space to chop
   # the string
   elsif (length($str) > $desired_len) {
      #print "here" and exit;
      $str = substr($str, 0, $desired_len);
      my $token = substr($str, $desired_len-1, $desired_len);
      while($token !~ / /) {
         $str = substr($str, 0, $tmp_len);
         $token = substr($str, $tmp_len-1, $tmp_len);
         $tmp_len--;
      }
      $str .= '...';
   }
   return $str;
}
[download]

Thanks in anticipation :)

Update: Thanks to all for your sharing of code :)

Update2: I now have a problem of deciding which method to go with but I've undoubtedly learnt new ways of looking at the same problem. Great thanks and cheers