No, that's not it. Your problem has nothing to do with thread.

Use «use strict; use warnings;»!!! $num_threads is undef when you execute the BEGIN block.

1. The file is compiled.
   1. my $num_threads = 2; is compiled.
   2. my $thr = []; is compiled.
   3. BEGIN { ... } is compiled.
      1. ...
      2. for (my $ii=0; $ii<$num_threads; $ii++) { is compiled.
      3. ...
   4. BEGIN { ... } is executed.
      1. ...
      2. for (my $ii=0; $ii<$num_threads; $ii++) { is executed.
      3. ...
   5. print "done\n"; is compiled.
2. The file is executed.
   1. my $num_threads = 2; is executed.
   2. my $thr = []; is executed.
   3. print "done\n"; is executed.