string and variable concat in -f checks

[Boris Bartsyts]

hello,

is it possible to do something like this:

if (-f /cache$request_filename) {
    rewrite ^(.*)$    /cache$1;
}

it should check for the requested file in the /cache dir and if it
doesn't exist proceed to remaining rewrite rules. i can't make this
clause working, please correct it or point to other soultions.

Bob