I am sorry about nitpicking but setting $| to a true value will turn on auto-flushing for the currently selected file handle. I know it is being pedantic, but just because STDOUT is the default selected file handle doesn't mean it has to be:

#!/usr/bin/perl
use strict;
use warnings;

open (FILE, '>', 'foo.txt') or die "Unable to open foo.txt for writing
+ : $!";
select FILE;
$| = 1;                               # FILE is now unbuffered
print "hello, world\n";

print STDOUT "goodbye cruel world\n"; # still buffered
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