my $quit = 0;
my %tasks = (
   sub_1 => { code=>\&sub_1,
              state=>'idle', 
              deps=>[ ]
   },
   sub_2 => { code=>\&sub_2,
              state=>'idle',
              deps=>[ ]
   },
   sub_3 => { code=>\&sub_3,
              state=>'idle',
              deps=>[ 'sub_1' ]
   },
   sub_4 => { code=>\&sub_4,
              state=>'idle',
              deps=>[ 'sub_1' ]
   },
   sub_5 => { code=>\&sub_5,
              state=>'idle',
              deps=>[ 'sub_2' ]
   },
   sub_6 => { code=>\&sub_6,
              state=>'idle',
              deps=>[ 'sub_2' ]
   },
   sub_7 => { code=>\&sub_7,
              state=>'idle',
              deps=>[ 'sub_1', 'sub_2' ]
   },
);

sub sleep {
    # No work to do, so doze for a while
    ...
}

sub get_next_task {
   my $task;
   for my $cur (keys %tasks) {
      next unless $tasks{$cur}{state} eq 'idle';
      my $deps_not_ready = 0;
      for my $dep (@{$tasks{$cur}{deps}}) {
         if ($tasks{$dep}{state} ne 'done') {
            ++$deps_not_ready;
         }
      }
      next unless $deps_not_ready == 0;
      return $task;
   }
}

sub thread {
    while (! $quit) {
       my $task = get_next_task();
       if (defined $task) {
          $tasks{$task}{state}='busy';
          $tasks{$task}{started}=time;
          if (&{$tasks{$task}{code}}()) {
             $tasks{$task}{state}='done';
          }
          else {
             $tasks{$task}{state}='FAULT';
          }
          $tasks{$task}{finished}=time;
       }
       else {
          # No tasks are available right now
          sleep();
       }
    }
}