I certainly can't rule out some kind of OS or installation specific thing that makes your setup different than mine. I am using WinXP and ActiveState 5.10, although I know from experience this is the same on ActiveState 5.6.

Here is a simple test program. Try it on your machine and see if you get the same results.

#!/usr/bin/perl -w
use strict;
$|=1;  #test was run with and without this statement
       #this turns buffering off

print "this is a test\n";
print "this is a test2\n";
print STDERR "this is an error after line2\n";
print "this is a test3\n";
print "this is a test4\n";

__END__
Prints: without $|=1........
this is an error after line2
this is a test
this is a test2
this is a test3
this is a test4

Prints: Now with $|=1 ......
this is a test
this is a test2
this is an error after line2
this is a test3
this is a test4
[download]

The STDERR message gets printed first because the lines from STDOUT haven't gone to the screen yet.

Using $|=1 decreases I/O performance by maybe 30% or something like that. Normally of no consequence unless you are doing a lot of output or are trying to get the time sequence of things right.

Update: deleted few lines about my version of tee.pl as there is a similar topic going on in another thread and I got them confused. Re^3: Getting user input with STDOUT tee to file.