This is very weird. Using 5.8.0 (useperlio=defined) and 5.005 (useperlio=undef) on FreeBSD, the first snippet doesn't do anything. The second, however, works. Some playing around with filehandle duplication and copying (using the <&=FILEHANDLE construction) got me something that does work:

close(STDIN);
open(STDIN, "<&=DATA");
while(<STDIN>){
  print "STDIN: $_";
}
__DATA__
Foo
Bar
Quux
[download]

Of course this doesn't help explaining the problem, but it adds a rather strange case to it. Time for some pondering.

Arjen