if ($a) {
   foreach $a (@a) {
      print $a;
      discover ($a);
      }
   }

if ($b) {
   print $b:
   discover ($b);
   }

sub discover {
  ## do lots of repetitive stuff
}

##</code><code>##

if ($a) {
   my $manager = new Parallel::Forkmanager( 10 );
   foreach $a (@a) {
   $manager->start and next;
   print $a;
   discover ($a);
   $manager->finish;
   }

if (b) {
   discover (b);

sub discover {
   do lots of repetitive stuff
}