As tilly and toma pointed out, error checks might be useful; to quote approximately from Camel, 2nd edition:

"""
Always check the return values of system calls.
Always check the return values of system calls.
ALWAYS CHECK THE RETURN VALUES OF SYSTEM CALLS!
"""

Update: Well, that didn't seem to be the problem, because I added checks, and a wait (marked by ##):

$water = {
map
{
    my $process = $_;
    my @inputs = @ {$processes{$process}[1]};
    $_, {
    prerequisites => [ @inputs ],
    oo_output =>
        sub
        {
        my ($data) = shift;
        my ($pid, @fh, $active);
        @fh = map { $data->get($_) } @inputs;
                push @fh, \*STDIN if ((defined $start) and $start eq $
+process);
                $active = scalar @fh;
        local (*IN, *OUT, $/, $\);
        $/ = 1;
        $\ = '';
        open2 \*OUT, \*IN, $processes{$process}[0] or
            die "Can't open2 \"${\($processes{$process}[0])}\": $!, $?
+, $^E; stopped"; ##
        select(IN) or die "Can't select IN: $!, $?, $^E; stopped"; ##
        for (;;)
        {
            my $piece = join '', map { my $x; (eof $_) ? "\0" : (defin
+ed ($x = <$_>)
                                  ? $x : (
                                      die "Can't read: ", ##
                                      "$!, $?, $^E; stopped"
                                      )) } @fh;
            (grep { not eof $_ } @fh) ? (print($piece)
                         or die "Can't print: $!, $?, $^E; stopped") #
+#
            : (last);
        }
        close (IN) or die "Can't close IN: $!, $?, $^E; stopped"; ##
        return \*OUT;
            } } } keys %processes };

tie %river, 'Data::Flow', $water;
$SIG{CHLD} = sub { wait }; ##
{
    my $output = $river{$final};
    local $/ = undef;
    local $_;
    defined($_ = <$output>) or die "Can't read: $!, $?, $^E; stopped";
+ ##
    print or die "Can't print: $!, $?, $^E; stopped"; ##
}
exit 0;
[download]

C:\premchai21\perl>perl multipipe.pl -f pipes
-p destination: Broken pipe
-p destination: Broken pipe
#kill.#
#darn!  killed emacs instead#
#kill!#
C:\premchai21\perl>

process cat perl -spew
start cat
final cat

C:\premchai21\perl>perl multipipe.pl -f tiny
hello, world!
#^Z, RET#
#^Z, RET#
#^Z^Z^Z^Z^Z^Z^Z^Z^Z#
 at multipipe.pl line 102.

C:\premchai21\perl>

Anyone?