# do this once (obviously, I hope)
use IO::File;
open LOG, ">>log.out" or die "opening log.out: $!\n";
LOG->autoflush(1);
printf LOG "PROC $$: %d %s run starts %s\n",
   time, __FILE__, scalar(localtime);

# put lots of these throughout your program
printf LOG "PROC $$: %d %s(%d)\n", time, __FILE__, __LINE__;