Something like this may do what you need. Instead of 'switching' tee on and off, use two filehandles.

# unbuffer STDOUT
$| = 1; 

open(TEE, "| tee $filename") || die "Can't open pipe: $!";

# unbuffer output to the pipe.
select(TEE);
$| = 1;
select(STDOUT);

print TEE "some stuff that goes both places\n";
print "some stuff that goes to the browser\n";
print TEE "some more stuff that goes places\n";
[download]

I've not tested this, but it should do the trick. If you get output in the wrong sequence, the some of the output is being buffered somewhere along the line. Setting $| should keep that from happening...

/\/\averick