From what I understood it's implemented on the Perl side as XS-code inlining JS code and returning proxy objects.

There where various examples how it looks like in this thread.

e.g. in Re^2: Run Perl 5 in the Browser!

js('document')->getElementById('my_button')
    ->addEventListener('click', sub {
        js('window')->alert("You clicked the button!");
    } );
[download]

so you could interpret method chaining as a "DSL" (at least Fowler does)