You can split the fork and the exec up if open mashes them together too much.

pipe CHILDREAD, CHILWRITE;
defined( my $pid = fork ) or die "fork: $!";
if ( $pid ) {
    # read on CHILDREAD;
} else {
    open STDERR, ">&CHILDWRITE";
    open STDOUT, ">&CHILDWRITE";
    exec( $somecmd );
}
[download]

This is precisely the type of plumbing that a shell will do when you say 2>&1, except without the unportable syntax ;-)

That said, IPC::Run and friends already abstract all of this out, so there's no need to reinvent the wheel.