package My::RouteHandler;

use Dancer;
use Path::Tiny;
use Data::GUID;
use Time::Piece;
...

post '/some/route' => sub {
    my %args = params;
    if ( $args{'long_task'} ) {
        my $guid = Data::GUID->new->as_string;
        my $url  = '/files/results/' . $guid . '.html';
        my $path = config->app_root . '/app/restricted' . $url;
        my $file = path( $path )->touch;
        
        fork and do {
            $file->spew( sprintf "Results not ready. Please reload this page in a few minutes.<BR>URL: %s<BR>Created: %s", $url, gmtime->datetime );
            return redirect $url;
        };

        # now we are off line
        my $stuff = do_some_lengthy_task();
        $file->spew( $stuff );

        exit; # because we already forked and returned to the browser
    } else {
        ...
    }
};

1;
__END__