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

Notes:

• No guarantees are made that the sleep will wake up when desired.
• Doesn't make sure that $target >= time().
• If you catch signals, sleep can be interrupted prematurely.

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