What you probably want to do is set up a signal handler for $SIG{CHLD} which uses wait() to clean up the child. Since it's in the CHLD signal handler, it will only get called when a process has already finished, and so it won't really have to wait. Here's some sample code:

#!/usr/bin/perl

$SIG{CHLD}=sub { wait(); print "Child exited\n";};
$| = 1;

while(1)
{
  if ((my $f = fork()) == 0)
  {
    # Child
    sleep(1);
    exit(0);
  }
  else
  {
    print "Started child $f\n";
  }
  sleep(2);
}
[download]