for $foo ('a'..'c') {
    open FH,'|-', 'cat -';
    select FH;
    for $bar (1..rand 10) { write }
    close FH;
    $- = 0;
}
format FH_TOP = 
=@=
$foo
.
format FH =
 @
$bar
.
__END__
=a=
 1
 2
 3
 4
=b=
 1
 2
=c=
 1
 2