And for FreeBSD 6.1 and Perl 5.8.8, I get

pid is 18693
No error
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The error occured in the forked child process, so the exec or die would get sent to the file handle piped to $ERROR, and only the child's $! would be affected if the fork succeeded.

ikegami erases the paragraph he was writting.
Oo! I just figured it out!

The OP must be on a system where fork is used ($^O eq 'os2' || $^O eq 'MSWin32' is false). The fork succeeds, but the exec fails. He's getting output from both the child and the parent! Fix:

my $pid = eval { open3($WRITE, $READ, $ERROR, "$command") };
if (!$pid) {
   # We're in the child.
   # The fork succeeded, but the exec failed.
   POSIX::_exit($!);
}
...
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Update: Oops! Looks like almut beat me to it here