The posted code returned:

Undefined subroutine &threads::shared::_id called at math1.pl line 5.
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Testing is done with 5.8.4 on windows XP.

Update: The above code could be fixed like this:

use threads; #added
use threads::shared;

  my $var : shared;

  if (threads::shared::_id($var)) {#pass the var not the ref
      print("'\$var' is shared\n");
  }
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