You can't get there from here

"Is my program going to finish?"

Sometimes it is really hard to tell:

        sub do_i_finish_or_not_huh {       # actually nobody knows
          my $arg = shift;
          until ($arg == 1) {
            if ($arg % 2 == 0) { $arg /= 2 }
            else               { $arg = $arg * 3 + 1 }
          }
          return 1;
        }