open STDERR, ">", "/path/to/log";

##</code><code>##

open STDOUT, ">", "path/to/log";

##</code><code>##

#errorToOut.pl
use strict;
use warnings;

open STDERR, ">>", "log.txt";
open STDOUT, ">>", "log.txt";

#outputted to stderr
warn "warning!";
#outputted to stdout
print "Test.";

#Log.txt winds up looking like:
#warning! at errorToOut.pl line 9.
#Test.