After a little prod from anon monk I read the docs a little closer and found my answer in Plack::App::File

enable "Static",
    path => sub { s!^/static!! },
    root => './html-files',
    ;
[download]

My mistake was thinking PM::Static behaved the same as PM::Pod - when i looked at the source of both i saw that PM::Pod was doing the substitution itself:

Plack::Middleware::Pod

my $r = $self->root || './';
$path =~ s!$path_match!$r!;
[download]