But there are example code in perldoc, which make me think it's sometimes buffered

line-buffered means it buffered, so the article says its line-buffered, the code in select shows how to unbuffer it -- do you get it now?

In my tests it's always line buffered, even when not attached to terminal, but I wonder if this is always true or not.

Here is a start in finding out, it depends on your platform

UTSL :) http://perl5.git.perl.org/perl.git?a=search&h=HEAD&st=grep&s=stderr.%2Bbuf&sr=1
http://perl5.git.perl.org/perl.git/blob?f=Porting/Glossary#l1937

1937 d_setlinebuf (d_setlnbuf.U):
1938         This variable conditionally defines the HAS_SETLINEBUF sy
+mbol, which
1939         indicates to the C program that the setlinebuf() routine 
+is available
1940         to change stderr or stdout from block-buffered or unbuffe
+red to a
1941         line-buffered mode.
1942
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