open HANDLE, "some_script |";
push @INPUT, *HANDLE;
while (<@INPUT>) {
  if ($some_condition) {
    open HANDLE, "some_other_script |";
    push @INPUT, *HANDLE;
  }
  do_something_with_input();
}

##</code><code>##

use IO::Select;

$select = IO::Select->new();
# repeat next line for all filehandles to poll
$select->add(*FILEHANDLE);
if (@ready = $select->can_read(0)) {
    # input waiting on the filehandles in @ready
}