See Plack::Request methods "parameters" and "uploads". This should be very familiar to you if you know CGI.pm.

Server (this is one line):

plackup -MPlack::Request -MDDP -e'my $app = sub { my ($env) = @_; my $
+req = Plack::Request->new($env); p $req->parameters; p $req->uploads;
+ return [200,[],[]]; };'
[download]

Client:

curl -v -F somekey=somevalue -F upload=@somefile http://localhost:5000
[download]

HTTP trace:

POST / HTTP/1.1
Host: localhost:5000
User-Agent: curl/7.65.0
Accept: */*
Content-Length: 322
Content-Type: multipart/form-data; boundary=------------------------84
+f03d39abacb4c6

--------------------------84f03d39abacb4c6
Content-Disposition: form-data; name="somekey"

somevalue
--------------------------84f03d39abacb4c6
Content-Disposition: form-data; name="upload"; filename="somefile"
Content-Type: application/octet-stream

somefilecontent
--------------------------84f03d39abacb4c6--
HTTP/1.0 200 OK
Date: Fri, 14 Jun 2019 07:22:11 GMT
Server: HTTP::Server::PSGI
Content-Length: 0
[download]

Abbreviated server output:

Hash::MultiValue  {
    public methods (24) : …
    private methods (2) : …
    internals: {
        somekey   "somevalue"
    }
}
Hash::MultiValue  {
    public methods (24) : …
    private methods (2) : …
    internals: {
        upload   Plack::Request::Upload
    }
}
127.0.0.1 - - [14/Jun/2019:09:22:11 +0200] "POST / HTTP/1.1" 200 0 "-"
+ "curl/7.65.0"
[download]