builder {

    # [...]

    sub {
        my $psgi_env = shift;
        /^\/$/ and $_ = "" for $psgi_env->{SCRIPT_NAME};
        $psgi_env->{PATH_INFO} ||=
            trim( @{$psgi_env}{'SCRIPT_FILENAME', 'DOCUMENT_ROOT'}, ".html" );
        $poet->app_class('Mason')->handle_psgi($psgi_env);
    };
};

sub trim {
    my ($string, $head, $tail) = @_;
    $string =~ s{ \A \Q$head\E (.+?) \Q$tail\E \z }{$1}xmsr;
}