open (STDOUT, ">$log_dir/$process.log") or exit 1;
open (STDERR, ">&STDOUT") or exit 2;
flock(STDOUT, LOCK_EX | LOCK_NB) or exit 3;
truncate STDOUT, 0; #clear out STDOUT to start with a fresh log file.

print "This is my log file.\n";
qx!someprogramtorun!;

(do some more stuff)

__DATA__
(what I see in the log file)
This is my log file.