You are suffering from buffering. You also have at least two precedence bugs in the conditionals in your first statement.

use IO::Handle;

if (!open(TPOOL, "|thread_pool.test)) {
   print "error: $!";
   exit(1);
}

TPOOL->autoflush(1);

while(1)
{
   sleep 10;
   print TPOOL "This is just a test\n";
}
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