use IO::Select;
my $s = new IO::Select;
$s->add($h1);
$s->add($h2);
my (%handler, %buf);
$handler{$h1} = sub { ...handle a line from $h1... };
$handler{$h2} = sub { ...handle a line from $h2... };
while ($s->count) {
  my @ready = $s->can_read();
  for my $h (@ready) {
    my $nr = sysread($h, $buf{$h}, 1024, length($buf{$h}));
    if ($nr == 0) { # eof detected
      $s->remove($h);
    } else {
      if ($buf{$h} =~ s/\A(.*?)\n//) {
        $handler{$h}->($1);
      }
    }
  }
}