No need to use two threads. 

use IO::Handle qw( );
use Time::HiRes qw( time sleep );

open(my $fh, '>', $file) or die;
$fh->autoflush(1);

my $target = time() + 0.500;
for (;;) {
   print($fh "1");

   $target += 0.500;
   sleep($target - time());
}

[download]

If your program needs to do something else at the same time, you can use a dedicated process (fork) or thread (async{ ... }->detach();).

Notes:

Update: Fixed bug identified in reply. Moved autoflush out of the loop.

In reply to Re: Any ideas for using perl as a simple pulse generator? by ikegami
in thread Any ideas for using perl as a simple pulse generator? by bfreemer

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