Plack つかうとこういうの簡単にかけてよい。

use common::sense;
use autodie;
use Plack::Runner;
use Plack::App::Directory;
use FindBin;
use Filesys::Notify::Simple;
use Parallel::Runner;

my $docsdir = "$FindBin::Bin/../../docs/";
my $htdocs = "$docsdir/_build/html/";
die "missing $htdocs" unless -d $htdocs;

my $runner = Parallel::Runner->new(2);
$runner->run(sub {
    my $app = Plack::App::Directory->new({ root => $htdocs })->to_app;
    my $runner = Plack::Runner->new();
       $runner->parse_options(@ARGV);
       $runner->run($app);
});
$runner->run(sub {
    chdir $docsdir;
    my $watcher = Filesys::Notify::Simple->new( ['.'] );
    while (1) {
        $watcher->wait(
            sub {
                system "make html";
            }
        );
    }
});
$runner->finish();