Why no warning on reading from pipe opened for writing?

Given this:

$ ./perl -wle 'open $fh, ">", "/tmp/foo"; print for <$fh>'
Filehandle $fh opened only for output at -e line 1.

Why no such warning here:

$ ./perl -wle 'open $fh, "|-", "true"; print for <$fh>'

?  Doesn't Perl know that $fh is only for writing?

Verified on perl 5, version 13, subversion 4 (v5.13.4-144-gf1dcae2) built 
for i686-linux.

-- 
Peter Scott

• Why no warning on reading from pipe opened for writing? by Peter Scott