No, you don't understand how route parameters work.

Put ONLY this in a file called something.pl and run it with perl something.pl.

use strict;
use warnings;
use Dancer2;

get '/:site_id' => sub {
    my $id = route_parameters->get('site_id');
    print "$id" . "\n";
};

dance;
[download]

Then in a separate window, run curl localhost:3000/MY_TOKEN_VALUE (or load that URL in a browser).

In the first window you will see printed "MY_TOKEN_VALUE".

This is as simple as I can make it.

Where in your design will the site ID come from? Do you think that it is known to the app? Or to the caller?